thanks for the comments.

I didn't bend anything to get the seatback more upright. I just never changed it from the original setting that it was in when I got it (which was as erect as it would go, I think).

Any idea why it would just snap--on both sides simultaneously?

Also, I hang my backpack (with a laptop computer and other stuff) on the back for my commute to and from work. Could that have been what "put it over the edge"?