When a target dies while Yasuo is ulting that target, it'll freeze Yasuo in place until (I'm guessing) his ult would finish anyways.
It may just be on High Noon Yasuo as well. I couldn't get a chance to test it on the classic skin.
It seems you would follow these steps:
1. Play Yasuo
2. Get someone low enough to kill them with a spell or two
3. Ult someone, but have someone else get the killing blow during your ult
This seems to be how you get stuck for a second or two.
**Edit:** It seems like it's both Yasuo skins and a general problem with Yasuo