I get that sometimes. I have observed that it seems that a few different things can be the cause. Sometimes it's just because the width of the bevel and/or spine wear changes over the length of the edge. Sometimes it's because of a slight warp or smile. Sometimes it's because a part of the edge isn't quite there. And sometimes it seems to randomly happen.

In general, it seems to me that anything that changes the contact between razor and hone, or the amount of contact, can have this effect. Even very small scale changes.