Remote trigger flash use - sync curtain line showing (but <max sync speed)

I was trying to take some pics the other day using my Yongnuo YN622C mounted on my Canon camera with a 580EXII mounted on a Yongnuo YN622 as a receiver. I was only using a shutter speed of 1/125 (so well inside the camera's max flash sync speed), yet I was getting the curtain line appearing in the shot like I was exceeding the sync speed. I have been using the triggers for a while and don't recall having that issue before.

Any thoughts on what might have happened...?

 

I found the YN622 became unreliable when batteries got old - are they fresh/freshly charged?

 

I would have thought they should be OK as the triggers have not seen a lot of use and I have alkaline batteries in there. However, I'll definitely give that a try. Thanks for the thought.

 

at a complete guess. is HSS turned on? check if the flash (or the camera settings) is set to '2nd curtain sync'

 

Are you suggesting I should have HSS or 2nd curtain sync on, or shouldn't have them on? I'll have to check my settings, but I think HSS was turned on. However, my shutter speed was only 1/125th or thereabouts. (Does having HSS turned on when it is not required actually mess it up?)