Here's an analogy I sometimes use...
A note starts with a little wobble and then settles down and steadies. This sometimes makes me think of a frisbee. Notice that, when thrown, a frisbee sometimes wobbles a bit before settling down.
If only we could get right to the steady tone and bypass the wobble.
Why does the wobble happen? Well, I'm not sure there's one reason. I notice this more often with bass trombonists.
On lower notes, I often see students blowing with an air stream that is too fast/forced. Other times it seems as if the lips are trying to buzz the wrong pitch and need to be "fixed" into place by the instrument.
How to fix that wobble? Buzz some notes and make sure they're centered. On the instrument, try some breath attacks. Make sure you don't use an explosion of air to start a note.