Why did my roasting bags pop?

[Shinyshoes1]

I put some fish in one of those roasting bags and in another tomatoes. Both of them popped and all the juices ran all over the tray.

I thought the idea of them was to have food cooked in it?

Where did I go wrong?

The roasting bags I get, according to the packet your have to pierce the top a few times :)

ahhhhhhhh I see I will have to have a read, because it's a tie up bag I just assumed it was meant to be airtight to seal in moisture