Your intervals aren’t oo long to stay on one diagonal but most riders tend to pick up the same diagonal a LOT unless they intentionally think about it. In other words, either the rider has an unconscious favorite diagonal or the horse is smooither to the rider on one diagonal over the other so the rider without thinking begins a trot on that more “comfortable” diagonal.
When riding trails at a trot I change diagonals every time the trail bends to a different side. So that I keep using both of them. If I’m trotting a long straight section I intentionally switch diags and try to get a feeling of straightness in my body and horse to keep in check the normal human tendency to become crooked.
I also practice changing diags when up off the saddle instead of “sitting a bounce” as it takes more focus and balance to “stand a bounce” than to sit a bounce. I’ve noticed that a lot of riders tend to sit a bit hard as they change diags. A dressage instructor caught me doing that years ago as I changed directions across the arena diagonal. As I changed diags I sat down hard and my horse hollowed and rushed the next steps. It was me, not the horse who caused this. Just something to think about!
Bonnie S.