Sounds like the horse is simply going correctly at that point in time, which is a good thing!
I'm curious though, why haven't you asked the instructor what's going on? They are there first hand watching and coaching as this is happening, they will be able to tell you if the above really is what is going on.
It happened either when my instructor wasn't there or she was engaged with someone else. It seems I ride better when she is not around :)
Just kidding, she is great and really helped me a lot. We are trying to get me more independent, that's why I'm riding on my own/her not paying attention. I'm not very confident and I was very fixated on my instructor to do anything so we came up with a plan to put a stop to that.
I didn't ask her about it afterwards because I'm over my self-imposed "stupid question quota" for the month. It seems I'm using the forum to catch the overflow, sorry guys :)