This talent has multiple triggers and casting a shield actually triggers it a LOT less than the other. Casting a trap, by anyone or any pet, on the person with the shield triggers this skill a lot more often than the simple casting of a shield. You'll find it triggers a LOT when you cast feints on people even though they have no shield. The pet just tries to cast and nothing happens, same as if you cast a pierce on someone who has no shield. It's a handy talent in some cases, but you shouldn't rely on it. It's just a lucky bonus when it does occur.
May, wonderful word, and after the whole pet healing 'tweak' the meaning has become very, very broad sweeping. Personally when I see "may cast", a mental image of one maybe two developers messing with the settings watering it down to "once a week or so", about the same rate my former pet threw its "may cast unicorn".
Useless, welcome to the closet, sell ya later.
However, for stats they did not mess with (if that exists), there are apparent "triggers", and it sounds like your pet takes the throwing of a trap as a sign that its time to pierce. Placing a trap would be the trigger.
If it doesn't cast, don't be shocked. Solid stats are hard to beat. At least you can rely on them, except for the whole critical thing, don't get me started.