A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

You can activate Trench Behemoth's first ability even if it's already untapped and/or already has hexproof.

If the target creature can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.

If the target creature does attack, its controller still chooses which player or planeswalker it attacks, as normal. It can attack opponents who don't control untapped Trench Behemoths waiting to crush it, if any are available.