How did mulching kill my tomato plants overnight?

So at 6pm that day, I added some mulch around some of my tomato plants, maybe half a dozen or so, out of maybe two dozen total. The next day, every single plant that was mulched was dead or dying. I couldn't believe it. I tested with 3 different materials of mulches as well, from straw to dead grass roots to the fluffy bits that hang off trees. Every one that was mulched with whatever material, died. Every single tomato plant that wasn't mulched lived. I barely mulched as well. It dropped low to 38F that night as well, when usually its in the 40s and 50s.

Now obviously the mulching killed them, but how and why? Could someone explain the science behind what happened?

What was your source for the mulches? If the source was using certain herbicides, those would easily have affected the tomatoes.

What did the plants look like?
Leaf coloration, curling , wilting, stem cut off at ground level, etc.

perhaps the mulch encouraged cut worms to come to the surface?

I don't know, they just died so quickly. Just reduced to the stem. All the leaves withered and dead. Even with just using the dead grass from the very same ground. Other plants mulched before, for weeks, were fine. Kale and arugula and so on. They were Indigo Blue Berry Tomatoes if that matters. It's inexplicable. So strange. Eh, maybe we'll just never figure this one out.

I'm pretty sure your mulch insulated the soil, so that no warmth radiated upward during the night. So, with an ambient low of 38, the heat went up into what I guess was a clear, still sky, and your plants froze. I know it seems inexplicable. I've managed to freeze tomatoes with a good heavy mulch with lows well into the 40's.
My policy now is bare soil for tender plants until it's almost proper summer....trees well leafed out and no chance of lows much below 45-50.
As an aside, while researching this issue I found a scheme to make ice at a low temperature of 50 degrees. In a dry climate, lay a mattress on the ground under the open sky. It must be a clear night with no wind or fog. Place a shallow tray of water in the middle of the mattress, and in the morning you will have ice! Go figure....

Incredible. You are amazing. CASE CLOSED!

Today I learned - there is a time to plant, and a time to mulch.

Whoa whoa whoa whoa whoa. How do you make ice with a mattress? Is this witchcraft?

If the temperature change was sudden, the plants could very well have frozen, even with temps well above freezing.

If heat loss is rapid, water will freeze well above 32* F (0* C). (I have seen it happen in the 40's.)
It sounds like your mulch kept the heat in the soil from protecting the exposed parts of the plants.

My tomatoes died too after I put mulch around them, I noticed that most of their stem was rotting and it might be because it's hot and that there is no ventilation. Can anyone confirm, please? I am a newbie at gardening. FYI, my mulch is fresh wood chips.