Rax and I just came from trying to reproduce it, and I think we both agree that it came from spamming the skill buttons.
It's happened to any of our skills in any area of the map, and happens when we spam buttons.
I think it's more evident on buffs since they have a much longer cooldown time, compared to damage skills which only have a few seconds.
Rax's Blast Shot / Break Armour wouldn't work, while my buffs got the same.