I'm not professinoal of this field but have read into this crusade. I'll try to answer it to the best of my abilities.

To answer this question you must first confront two competing theories of the role the Venetians played in 4th Crusade. One is that they were the puppet masters who from the start didn't want the crusaders to actually attack the lucrative trading ports in Muslim lands and instead led them to attack Constantinople, who at the time where having a falling out with. Or two through a series of unfortunate events, of which a few very key ones were caused by the Crusaders themselves and the Venetians had no possible way to have influenced, led them to Constantinople.

If you examine the 1st theory you find faults and you can begin at the start of it, the crusader leadership negotiated with the Venetians for construct of a large fleet of ships, sailors, supplies, and transport to be Egypt. The first 2 crusades had shown that a land passage was a long and treacherous choice and they also picked up on what Richard in the 3rd crusade wanted to do, make a more stable crusader state in the holy land which first required the capture of Egypt. This was a huge endeavor and would require the Venetians to focus on the production for one year. This of course required a huge payment, to meet it. The crusader leadership decided that that once the crusading army of 33,500 gathered they would all pitch in for payment.

Of course only around 12,000 showed up, the crusaders were in debt and Venetians were in a tight spot as well since they had focused on this payout for a whole year and had given less priority to other commercial activities. This left the crusaders stranded and the Venetians then suggested for the time being they could help them with their problem with the city of Zara. For "the Venetians were the masters all along theory", this crucial part the crusaders misjudging actual support of the crusade is essential and well outside the hands of Venetians to influence.

The other crucial event that happens that leads to the road to Constantinople is outside of Venetian hands is Boniface of Montferrat the main leader of the crusade visiting his cousin Philip of Swabia during the attack of Zara, where he meets Alexios IV Angelos the son of deposed and blinded Isaac II Angelos. As to why he was off visiting while the rest of the crusaders went off to attack Zara is unclear but he managed to avoid the mass excommunication that pope had given to the crusader army that did participate in that event. Alexios offers not only to pay the crusader's debt but even more if they help him claim the throne of Constantinople. Crusader leaders agree to it and things go on to the sack of Constantinople.

Of course Doge of Venice, Enrico Dandolo, didn't have completely clean hand in this. Before becoming the doge, he had been the Venetian envoy to Constantinople and should of known how unfeasible were Alexios' promises politically and economically. He seemed more like he always had in mind of recouping Venetian expenses of the crusade and any other gains that could be derived from having an army and then an emperor indebted to Venice.

1st theory has been around since the aftermath of the crusade and 2nd theory is a more recent take of events. There are many other things that I haven't mentioned lend credence to both sides but I personally lean the 2nd Theory.

Some sources about the 4th crusade and by who add further light to this topic.

On the Conquest of Constantinople by Geoffrey of Villehardouin. Geoffrey of Villehardouin own account of the crusade. Can't really take him at his word since he was actually in commanding position in the crusade. His narrative pushed responsibilities of the events to others and down plays the role he himself has in the negotiations with the Venetians. Interesting aside his account was one of the first written narrative histories in French and not in Latin.

The Conquest of Constantinople by Robert de Clari Robert was a crusader of lower standing than Geoffrey in the crusade who sheds some light from a different perspective. Can't take him either at face value but does show that leadership didn't reveal quite a few things to most of the crusading army, like their destination of Egypt or that they had been excommunicated by pope for the Zara venture.

The Fourth Crusade - the Conquest of Constantinople by Donald Queller and Thomas Madden This is a modern history of the crusade goes into many of the details such as the primary sources and key characters some of whom I haven't mentioned.

Sorry for any spelling errors or so, I wrote this during a bout of insomnia.