Tripathi drew suspicion for being missing, in the area, and possibly for being brown. Granted, the images of Jahar and Tamerlan Tsarnaev were grainy, but Tripathi doesn't look a lot like either of them. Here's Sunil:
And here are the Tsarnaev brothers:
Sunil Tripathi's death doesn't appear to have anything to do with the Boston bombings or the accusation against him. He was apparently very depressed, possibly to the point of being suicidal, and it is this that likely caused his death. The moderators of the /findbostonbombers subreddit issued an apology to Tripathi's family for any duress they caused for the accusation.