when you turned your cards over were they directly in front of you, or (like i see many times) FLIPPED them over to land face-up next to your chips - over the line. Then some would argue that you voluntary mucked your cards
“When you’re down on your chips, pick your spot and push (then adjust figures to win more). In fact I can’t remember the last time I was ahead post-river when I have done this; unfortunately the rest is up to the gods” A. Mason 2007