Watching reactions on youtube and the overwhelming opinion for the seemingly knowledgeable (riders and ex-riders) is that the DQ was a terrible over reaction.
There's basically two questions - did he fail to keep his line? And did he elbow?
did he elbow? Almost everyone agreed that the elbow was caused by getting hooked by cavs bars and/or a reaction to maintain balance. And seems unanimous that the elbow didn't cause the fall as he was already going down.
did he fail to keep his line? Opinion seems to be split here. Some seem to think THAT was the wrong Sagan committed here (that he was too aggressive in taking cav off the line). Others seem to think "that's sprinting" and that his move was no worse than you see all the time.
Regardless, again, all seem to think the DQ was ridiculous. Apparently taking an over aggressive line (ie, the only thing he MIGHT be guilty of) is a punishable (eg, points) but not a DQ worthy offense while elbowing an opponent to the ground is clearly DQ worthy (but NO ONE thinks the elbow caused cav to fall).
So basically the riders and ex riders all seem to be scratching their heads about this decision