It didn't cause the fumble. There wasn't a fumble.
1) Because he wasn't touched before catching the ball, his knee/elbow touching didn't make him down.
2) Because he went to the ground in the process of making the catch, which is the second-most-debatable part of the ruling, his possession of the ball has to "survive the ground" in order for the catch to be complete, even if he's already in the end zone. If he had taken maybe a few more steps before crossing the goal line, such that he was clearly not still in the process of making the catch (and had thus become a runner), it may not have mattered what happened after he crossed the goal line. But they ruled that his catch and his hitting the ground were related.
3) The ruling that he lost possession of the ball as he hit the ground seemed a bit shaky to me, as a Pats fan. At absolute best (for us), it was reeeeeeeeeally close. Looked like he still had it wedged between his wrist and other hand. Mere contact with the ground does not mean it's incomplete if he never lost possession - the ruling held that he lost possession. That's the part I would focus your rage on.
It's the same as the Dez Bryant playoff catch/non-catch situation from a few years ago (edit: as I now see other people have likewise observed). The look of it, to the proverbial "50 people in the bar", was (A) possession, (B) crossed the goal line, (C) hit the turf but he was already in so it doesn't matter. But since the NFL rulebook has to draw bright unambiguous lines, even if they don't entirely make sense, it's a counterintuitive result. And even to arrive at that result, they had to decide two things (the italicized bits above) that really could have gone either way.