Snail shell triggers when their shield breaks. So the position you arrange your zero energy card is important. In 1v1, the closest guess you can make is based on the number of cards and energy they have left, most of the time a pure reptile termi puts up 115 or 145 shield. My bird does 120 damage, so if I predict wrongly I waste one energy because of the stun. Midline termis are more difficult to predict because you have no idea how many cards the backline axie draws. Have never seen a random trigger before. Maybe your opponent had a lot of shield put up? Try and pay attention to when the shield breaks next time you come play against snail shell, the stun comes right after you break that axie's shield.