Canter to trot transition? Why does this happen?

The first poster explained it perfectly. The horse always finishes it's stride before breaking to a trot. If their lead leg is forward, then their first trot stride is with their outside leg. This is why if you were to try to come up on the wrong diagonal, you would have to sit a beat to pick it up.

ok well when u canter the leading foot is to the inside. shes striking out with her inside foot. when u go down to a trot the outside foot comes first (i think) and u go up as soon as u feel her trot and ur on the right diagonal. i think its nice that that happens for u

i am 95 percent sure this is what/why it happens

It's a good sign that you have remained in balance through the transition. It's likely that when you land on the wrong diagonal, you've had a bit of a wobbly transition.

i think its just a 50/50 chance that you land on the correct diagonal, because you don't always land on the correct diagonal but sometimes you do.