My shoe almost got eaten by the buss stairs! Like how!!!

So I will just stood up from my seat when the buss stopped and then I went down the stairs and then SUDDENLY my shoes's boot heel or the back part of the boot just got stuck in between the staircases thin metal plates Like how?!  Luckily I didn't fall but just too a big step onto the ground and  from then just somehow it wiggled out by just three second. But honestly hopefully nobody saw it.