Not being a Q400 pilot myself, this is just a guess based on the workings of my own type - Perhaps on the Q400, as on my type, the spoilers automatically deploy when on the ground and travelling faster than a certain speed, as would be useful when having touched down. So in your case, I'd imagine it's due to taxying a little briskly. I imagine they retract both when the speed drops below this threshold, and when the thrust (or should that be power..?) levers are advanced beyond a certain TLA, to prevent you inadvertently flying about with thrust applied against them.