I'm in agreement with the other two answers you got and would add a third reason it might have happened: your stay history was considered robust enough, especially at the end of the annual period to have you comped into Platinum Status. The rules say 75 nights, but there are and have been exceptions.
Several years back I had a conversation with a Marriott Rewards associate who mentioned that in days gone by, there were plenty of PEs, as in Platinum Emeretus status given out, and not with the three million points, 12 years in the program, and 1000 elite night stays as required. Now (as in two years ago) there was, this associate said, a clamp down and PEs were not given out unless the three requirements were all met.
So, there are times when the rules are the rules and other times when they are bent--as in your case. But that's a good thing--enjoy that status and the 50% points bonus.