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