Hey,

I ran into this situation while playing with a friend yesterday. His Cavern Raider destroys my shield and hits a shield trigger, Locomotiver. Which effect activates first, Cavern Raiders creature search or Locomotiver's discard ability?

Thanks.

Cavern Raider's ability activates before you even choose what shield to attack, actually.

It goes like this: You declare you're attacking your opponent with cavern raider, and he can choose to block the attack. If he doesn't, activate his ability. Then, you choose a shield to break, and if it's a locomotiver, your opponent can play it for free. So yeah, the order is raider's ability -> loco.

It's just that 99.99% of the time it doesn't matter that the shield is chosen after attack effects, blocking etc. so people just say which shield they're attacking from the get-go.

Yes, this is the case here obviously. You use cavern raider ability before the the shield is broken. Your opponent must wait until you resolve the ability of Cavern. But anyways, I understand where you want to go with the question: "the priority of the effects". Well, the answer is simple, the turn player always have priority. Ex: It's my turn, I summon Magmarex, while my opponent has Quixotic Hero Swine Snout in his battle zone. There are 2 effects that seems to activate simultaneously, but it's my turn so Magmarex ability will resolve 1st destroying Quixotic, before it receive +3k power. On the other hand if it's my opponent turn and he attacks me with quixotic and trigers Magmarex from shields, Quixotic ability will resolve 1st, so I won't be able to destroy his quixotic with my Magmarex beeing triger from shields cause his quixotic already has 4k.