Weird. Mulch touching the plant wouldn't kill it in four hours. I suppose you or your neighbor didn't apply any herbicides? Does Round up even work that fast? You could have the mulch tested but that would be costly.
Why don't you do an experiment. Take a sacrificial plant in a pot and mulch it with your new mulch and see what happens. Use gloves.

I have never heard of mulch killing anything unless it smothered it, or caused rotting. I suppose that something could be in the mulch, but I'd be more suspicious of a herbicide someone (neighbor, spouse, or township) applied and it drifted onto your plants. It is very unusual to have many types of plants die within a very short time unless they were zapped. I think the mulch was just an unlucky happenstance.