I remember reading that the guy behind the original plotline left Bioware after EA bought the company, then the Mass Effect 3 script got leaked several months before release so the ending was hastily rewritten.

I’ve also heard that due to said leak the rewrite was done in a locked room by just two people, a writer and a producer, with no input or feedback from the rest of the writing staff. Given how abrupt and out of place the ending is (and what we have now is the expanded, retconned version - it was much worse at launch), I can easily believe that rumor.