It just did.
I was putting them back on hoping they wouldn't be mean again and the bottom one was fine. But the top one wouldn't go on so I pressed it up hoping it'd fit... then it suddenly clamped into place and I had a panic attack trying to get it off and ran to dad and dad ran around trying to find something to pry it out with and I managed to pull it off again after nearly screaming.
I hate these retainers... I'm gonna see the dentist on Monday and I refuse to wear these, even if my teeth'll get all screwed up again.