The word "trigger" is not really useful in this situation because catabolism is not triggered, it just means muscle breakdown is happening at a higher rate than muscle synthesis (both of which are constant). This happens under a lot of different situations, and can temporarily go back and forth a lot. You certainly enter net catabolism long before your fat reserves are depleted (else cutting would be simple: just stop eating until you're pure hard muscle).

There are ways to temporarily spike muscle synthesis, and one of them is eating a protein rich meal. This specific mechanism has a refractory period in the order of magnitude of hours. Intermittent fasting robs you of the ability to spike muscle synthesis as many times per day as you could otherwise.

This effect is not massive. Your body is pretty good at optimizing this stuff. But when you are cutting, in particular, or trying to bulk without putting on significant fact, these minor optimizations can add up.

Fat reserves won't help prevent muscle protein breakdown. The only way to help prevent muscle catabolism is by consuming protein, which if you are fasting you aren't doing. If your goal is to lose weight while maintaining as much muscle mass as possible it doesn't seem like IF would be the optimal choice for that. Its not physiologically possible to have muscle protein synthesis exceed muscle protein breakdown unless you consume protein. The body cannot produce essential amino acids and must obtain them from the diet.

Supposedly IF works badly for women because they enter catabolism more quickly than men. I think I even read a study about it - of course I can't find it now.