I have not insane testing but I think the following cause it:
Skill: Phalanx Doctrine + Shield with +movement speed on full shield
Every time I fight small monsters and my shield goes up without taking damage, I still have just the normal boost, but when the Phalanx Doctrine stacks run out, I get boosts it seems for each stack that disappeared. The movement speed boost disappears when I take damage again.
It’s easy to test out, and it’s possible to stack by destroying the truck on the rooftop near Roland’s Rest over and over possible up to 9 stacks. There are probably instances where you can get higher than 9 without taking damage.
Also, the boost stays when entering a vehicle and teleporting somewhere else.