it seems an intermittent bug, generally I’ve been able to replicate it whilst in melee combat with two or more monsters. Holding and quickly releasing (slightly longer than a tap) the ability key sometimes has the Shield Rush movement continue. You’ll continue moving until you hit an object or run out of mana - this can be quite a distance if you miss the walls!
It does not seem to happen versus single monsters in melee and seems to need a group to trigger the bug,.