There has been a huge shift in academia on the subject of Sekigahara since about 2010. Basically everything to do with this question is false.

So, first, it is quite likely Ishida Mitsunari was not the leader of the so-called "Western Army." Certainly he was never the nominal leader (that was Mōri Terumoto) and certainly he had great influence. But that's a far stretch from him being the de-facto leader, and even if he was, there's the question of how much influence he actually had. Consider that although the main Mōri contingent was not at Sekigahara, the largest contingent in Ōgaki was not Mitsunari's but that of Ukita Hideie, by far. According to Mitsunari's letter to Sanada Masayuki, Hideie mobilized 18,000 men to Mitsunari's 6,700, and Ukita Hideie was both overall commander in Korea for the 1592 expedition and a member of the counsel of five elders. Now it's highly possible Mitsunari was exaggerating numbers to Masayuki, and the Ukita clan had ran into an internal crisis that likely led to Hideie having a far smaller contingent than planned. But even if Hideie brought just half the planned number while Mitsunari himself brought 6,700 as in the letter, the Ukita contingent would still significantly outnumber Mitsunari's. Not to mention the Mōri contingent on Mount Nangū would've also numbered 15~20,000. Even if Mitsunari was de-facto commander in the Mino sector, it's hard to think he would've had much direct control under such circumstances, and any decisions he made the others must have at least agreed to it, so Mitsunari's personal popularity mattered little. After the defeat, the surviving clans would very much wanted to use him as a scapegoat to insure their own survival, and him being a convenient scapegoat certainly helped the Tokugawa as well.

The story of Mitsunari's unpopularity leading to chaos in the ranks during the campaign comes from the story of Mitsunari shooting down Shimazu Yoshihiro's idea of a night raid prior to the battle, leading an angry Yoshihiro to sit out the main battle with the Shimazu contingent. The idea of a night raid could not be found in any contemporary or early-Edo records and so is almost certainly a later fabrication. Based on what we can glance from letters and surviving memoirs of Shimazu clan members who were at the battle, the Shimazu didn't sit out because Yoshihiro was angry. They sat out because their contingent was tiny, less than 1,500 men, and so was divided in two and held in reserve. And they didn't even really sit out because some of them were committed.

There were four confirmed traitors at the battle of Sekigahara, Ogawa Suketada, Wakizaki Yasuharu, Kikkawa Hiroie, and Kobayakawa Hideaki. Ogawa Suketada changed sides just to be on the winning side. Ieyasu actually punished him for it. Wakizaki Yasuharu never wanted to be in the "Western Army" but he was caught in Ōsaka when war broke out so was forced by circumstances to be take Mitsunari's side, since it was that or death. He had clearly been pro-Ieyasu after Hideyoshi's death, long before war even broke out.

Kikkawa Hiroie supposedly positioned his own contingent in front of the rest of the Mōri on Mount Nangū to block them from participating. Currently it's possible that this was not the case, in that the force on Mount Nangū didn't participate not because Kikkawa Hiroie blocked them, but because the main force at Yamanaka was routed in less than two hours. But let's ignore that hypothesis and go on the assumption that he did block them from participating. Surviving letters let us know that Hiroie was already in contact with Ieyasu over a month prior to the battle. According to Hiroie himself he was always against fighting Ieyasu, it's just that Mōri Terumoto listened to Ankokuji Ekei instead of him. In other words, Hiroie changing sides had nothing to do with Mitsunari. The Mōri was divided in two, whether to fight for or against Ieyasu, and Hiroie, leading the "for" side, just decided to stick to what he thought was correct even after the Mōri Terumoto decided otherwise.

Which bring us to the most important traitor, Kobayakawa Hideaki. Hideaki left us no surviving document to say why he switched sides. However, there are a few things to consider.

1. In 1598, while deployed in Korea Kobayakawa Hideaki was suddenly recalled to Japan for unknown reasons (it's often said it was due to Mitsunari reporting his misconduct in battle in Korea, but no surviving contemporary source says so). He was then ordered to transfer his 300k koku domain in northern Kyūshū to 150k in Echizen, effectively a demotion, with northern Kyūshū coming under Hideyoshi's direct control under Mitsunari's administration. The most likely reason put forth for this transfer was to better organize northern Kyūshū to supply Korea. In other words, even if it was Mitsunari's idea (and it could very much be Hideyoshi's), it's really Hideyoshi's fault for the war in the first place. Of course Hideaki might have blamed (rightly or wrongly) Mitsunari for the demotion. After Hideyoshi's death, over a year before Sekigahara, the council of elders, of which Ieyasu was effectively leader, restored him to northern Kyūshū, so perhaps Hideaki thought he owed Ieyasu one. In either case this entire situation was decided long before Sekigahara.
2. Both Kikkawa and Kobayakawa were important branches of the Mōri, and had great relations with the Kuroda, one of Hideyoshi's leading vassals, during Hideyoshi's days. After Hideyoshi's death, the Kuroda also firmly sided with Ieyasu. We know it was the Kuroda who acted as the go-to for Kikkawa Hiroie to contact Ieyasu, and the Kuroda says it was they who convinced Kobayakwa Hideaki (and Kikkawa Hiroie) to switch sides so we know they at least played that role for Hideaki as well. Again this is not something Mitsunari could do about and was decided long before Sekigahara. The only thing Mitsunari could be at fault then was pissing off the Kuroda (and a bunch of others) to they point that they forced his removal from the post-Hideyoshi administration and sided with Ieyasu at Sekigahara. However, as far as we can tell this "unpopularity" was due to the conduct in the Korean war, mainly because Mitsunari acted as both Hideyoshi's eyes and ears for info and his mouth to relay unpopular orders. So really Mitsunari's "unpopularity" stemmed from him just doing his job and Hideyoshi making a mess of things. And, finally, just like the first point all of this was decided long before Sekigahara.
3. Hideaki arguably switched sides before the battle commenced and both sides knew it. In fact it was arguably Hideaki's move that kicked off the main battle. He suddenly showed up at Mount Matsuo, and without orders kicked out the place-holder garrison there. Knowing Hideaki had switched sides, Ōtani Yoshitsugu moved from Yamanaka to Sekigahara to counter Hideaki. This lead to the main eastern and western forces at Akasaka and Ōgaki to move to Sekigahara to support their respective side, leading to the battle.

TL;DR: Mitsunari's unpopularity did not lead to any betrayals during the battle of Sekigahara. It might have lead to many deciding to side with Ieyasu or betrayal before the battle, but that comes from circumstances which he (or anyone) could do little about. And it's quite possible he was not the commander anyway, and even if he was he likely did not have much direct control and any action taken was by consensus.