Issue is simple to replicate.
1# Target someone with the mouse till skill row comes up.
(up and down arrow slips though the different rows, side arrows move curser in skill bar.)
2# Move target to someone else with the mouse.
(up and down arrow now has no effect, side arrows switch targets.)

Same thing happens if you right click and move camera, and pretty much anything that has to do with the mouse.

It's a very annoying effect, when you suddenly have to use your mouse to manually switch skill rows in the middle of battle.