Probably mostly on high ping/High pop servers, if you try to go ADS while from a crunch position try to stand up, you can stuck.
The video should show better than (my very bad English) words.
It seem related with the crunch to stand action somehow.
In fact, crunch to stand action have often a strange bounce https://youtu.be/F454TBiH2wY
This bounce often, but not necessarily, appear when the ADS problem it about to happen.