As far as I could tell SOME of the mummies were set off my proximity (so it was a plus if you could lure some of Nadine's army close to them, as the explosions tended to one-hit them), but for the most part in the long tunnel bits without enemies it seemed to be a matter of "time for them to go off now just as you're passing through to keep you on your toes lol!" With an aggravating number refraining from exploding just so you never felt certain in your safety.

That said I've only done the sequence once, so my observations could be totally incorrect! XD

Chloe! More pictures, hon, you can do it! ...do it at least for Nadine's reactions!