For me this is weird because the truck was coming from Russia and there are checkpoints too, so Ukrainians would need to have somehow access to explosives inside Russia. In theory you would find more Ukrainian sympathy in Crimea and not in Russia, but I do not know that area of the world so maybe the level of incompetence in Russia is even larger then I thought.
It could be that truck, but it could be the Putin "clever" tactics to claim Ukrainians are terrorists(for some reason Russians started to use the terrorist word when a military target is bombed but when they bomb civilian apartments and malls it is just the old soviet incompetence excuse)