The answer is no one knows. It is one of the most talked about and enduring mysteries of the Sengoku.

Everything between psychological madness and conspiracy by the court/Hashiba Hideyoshi/Tokugawa Ieyasu had been proposed.

The most often given reasons (fief switch to an enemy territory, Nobunaga's disregard for his hostage mother, getting kicked at the celebration ceremony of the destruction of the Takeda) are all likely created in the Edo era, often as fiction. Most theories created in the current day are either based on these fiction or incredibly circumstantial evidence (if any are given). There are some very circumstantial evidence for the theories of 1) having to do with the Chosokabe and planned invasion of Shikoku and 2) to protect the Imperial Court from pressures from Nobunaga. But both are still quite weak.

Of contemporary sources, Luis Frois does mention him getting kicked by Nobunaga (but he does not mention the circumstances and questions the reliability of the story) and there are mentions of mistreatment in one or two other sources. According to Luis Frois though, the man was a cunning, devious, friequent liar and would do anything to please Nobunaga for his own advancement, and according to Ota Gyuichi it was out of personal ambition. So according to two of our most trusted contemporary sources at least it was personal ambition. Of course the two could be biased, and no one has any records of goals and plans and such. So really it comes back to we don't know.

EDIT: I thought to do a bit of elaboration. Although we don't know what were Akechi Mitsuhide's reasons for turning on Oda Nobunaga, we can say with some certainty what they weren't:

Reason: Taken from a chronicle written in the early Edo (with dubious accuracy) Nobunaga ordered Mitsuhide to host Tokugawa Ieyasu. Mitsuhide used rotten fish. Nobunaga angrily ordered Hori Norimasa to rehost, making Mitsuhide loose face.
Problem: According to Ota Gyuichi's Nobunaga Chronicles, Ieyasu was hosted by neither of them.

Reason: Taken from a mid-Edo fiction, Mitsuhide was ordered to exchange his territory (Tamba and parts of Oumi) for Izumo and Iwami which were still in enemy hands.
Problem: The source is fiction written a century later. There are no contemporary source for the order. Historians also point out that even if the order was real, it was not unusual in the context of the Sengoku and as an increase in his realm worth should be thought of as a reward, not punishment, and there should be no grudge which arises from this.

Reason: Taken from a contemporary rumors collection, Nobunaga was drunk at a festival and threatened Mitsuhide at spear point and made him drink.
Problem: Both Luis Frois and Luis de Almeida recorded that Nobunaga did not drink.

Reason: Taken from a late Edo fiction, Nobunaga ordered the execution of clan leaders captured by Mitsuhide, who held his mother as hostage, leading to her death.
Problem: There's no contemporary source mentioning this. According to Nobunaga Chronicles, the clan's castle had been under siege by Mitsuhide himself since a year ago, and the clan leaders fell in battle when in one final desperation they sallied out.

Reason: Taken from the same source, Nobunaga ordered the burning of Erinji because one of his enemies had hid there, leading to the death of 150 monks.
Problem: The burning was ordered by Nobutada (Nobunaga's son). Nobunaga himself was not present.

Just to finish up English Wiki. There are two problems with Akechi declaring his intention with that poem of his. Besides what seem like a weak interpretation grasping at straws the poetry result from the renka was given to Oda Nobunaga, who's also fluent in poetry. If Mitsuhide was announcing his intentions through poetry, he was running a huge risk of Nobunaga seeing through his intentions. It seems like a unnecessary risk to take. And from recorded communications afterwards it doesn't seem like anyone took his part of the poetry as an announcement of coup.

Besides personal ambition then, the only other thing recorded by contemporaries was that there are some mentions of mistreatment by Oda Nobunaga (but not what's mentioned above) and grudges. But exactly what they are (besides getting kicked) is not known, or even the extent of the grudges.

A possible reasons for grudges brought forward after WWII was that Mitsuhide was tasked with diplomacy with the Chosokabe of Shikoku when the Miyoshi was still Oda's enemy, to attack the Miyoshi from behind. Once the Miyoshi surrendered to the Oda, Chosokabe's continue pressure on the Miyoshi brought them into conflict with the Nobunaga, with Hideyoshi taking the side of the Miyoshi. In the end Nobunaga sided with Miyoshi/Hideyoshi and attacked Chosokabe. From this incident, some possibilities include that a decade's hard work and diplomacy by Mitsuhide was being thrown away, that Nobunaga sided with rival Hideyoshi, and that Mitsuhide was tied to the Chosokabe through relationships between their respective vassals. Possibility of conspiracy with the Chosokabe is also raised.

Of other conspiracy theories, the one by the imperial court is the strongest. The imperial court was pressured by Nobunaga to relinquish power and break rituals and formality. Nobunaga might have demanded the position of Kampaku or Shogun. It's possible the court conspired with him, or he by himself rebelled to save the court.

The problem with this is that the same sources and cases used to suggest that Nobunaga was at odds with the imperial court could also be interpreted as Nobunaga simply deciding to work outside the traditional order in a mutual beneficial arrangement and was actually supported by the court. There's really no concrete proof of any level of animosity between the court and Nobunaga.

Some have also rose as a possibility of Mitsuhide being a sort of Brutus, killing a tyrant. This is based on Mitsuhide saying he had stopped the Oda father&son's tyrannical rule when writing to other daimyos after the coup asking for support. The usual reasons given are to prevent Nobunaga from destroying the old order like the Ashikaga Shogunate, imperial court, and monks.

Imperial court we've already touched upon. As for the Ashikaga Shogunate, it's true Mitsuhide was instrumental in linking up his old boss Ashikaga Yoshiaki and Oda Nobunaga, allowing Nobunaga to march on Kyoto to support Yoshiaki's claim as Shogun. But when Yoshiaki first turned against Nobunaga, Mitsuhide sided with Nobunaga, so one has to ask why he would side with the now even more powerless Ashikaga Yoshiaki this time around.

As for the monks, the cases are given that Nobunaga's burning of Mt. Hiei and massacre of the Ikko buddhists struck a nerve with Mitsuhide. However, new evidence has surfaced that demonstrates that Mitsuhide was actually supportive of the slaughter at Mt Hiei. And indeed his treatment of the Buddhist sects in his own realm was not exactly kind.

Finally, these reasons could be lumped into "keep/restore the old order" depends on an interpretation of Mitsuhide as conservative idealist, one popularized in the Edo period and often used in fiction. However this runs contrary to several of his recorded actions and his description by Luis Frois. Indeed it's pointed out that Nobunaga hates conservative idealists, instead favors realists with ruthless efficiency. If Mitsuhide was a conservative idealist, he shouldn't have been able to rapidly rise in ranks under Oda Nobunaga, but he did.

Also brought up is a possible worry hanging over Mitsuhide that Oda Nobunaga might banish him once he demonstrate he's no longer of use, as Nobunaga had done to others. He did after all just failed in his dealings with the Chosokabe. Of course this is complete speculation.

One last interesting possibility is that according to Kouyo Gunkan, Mitsuhide had been secretly in contact with Takeda Katsuyori about the possibility of switching side. Though it came to nothing, Mitsuhide might have been worried that Nobunaga would kill him if found out. As Anayama Baisetsu, ex-retainer of the Takeda who had joined the Tokugawa, had traveled to Azuchi (Nobunaga's castle) before and was travelling with Ieyasu in the vicinity at the time, Mitsuhide might have been worried that he would be found out so struck first. This is not impossible, as according to Mikawamonogatari, Nobunaga had ordered the seppuku of Tokugawa Ieyasu's son for the same reason. However Kouyo Gunkan is the only source of this, and though a valuable source is known to have mistakes. As Takeda Katsuyori was on his last legs at the time, one has to wonder why Mitsuhide would think of betraying Nobunaga for the Takeda (same question is brought up for Ieyasu's son Nobuyasu. Both he and Mitsuhide had fought at Nagashino without hint of treachery). Personally I also wonder if it's true, why not just kill Baisetsu instead.

So at this point, the strongest evidence points to ambition, some kind of grudge and/or worries, or a combination of them. And once again, according to our most reliable contemporary sources (Luis Frois and Ota Gyuichi) it was ambition. However, neither tells us exactly what this ambition was.