Hmm, that something had to do with Jayadratha himself. He forcibly abducted Draupadi. This happened during the time in exile. JD lusted after Draupadi and hence when the Pandavas were not in their forest abode, kidnapped her to ravish her. The Pandavas learnt of this, gave chase and defeated him in battle, after routing his entire army. Bhima kicked him and shaved his hair off, and declared him a slave of the Pandavas.
Draupadi entreats on his behalf actually, seeking he be spared, for he was married to Dushala, the only sister of the Kauravas so as to not make her a widow.
Insulted by this he then prayed to Shiva, as mentioned in the posts above. I suppose JD got what he deserved, and his further carrying on the vengeance was in no way ethical or just. He was the one who started it all, so it should have ended when he was defeated. Instead, he took further umbrage and prayed to Shiva for the boon to defeat the Pandavas. When that was refused by Shiva, as it was well nigh impossible, he asks for a boon to at least hold them all at bay for one day at battle. Even then, Shiva says it is not possible for him to defeat Arjuna for even a day, but he would be able to hold off the rest for a day.
As posted by Nakeeran, Arjuna was otherwise engaged that day when Abhimanyu pierced the vyuha and was killed
