They are beautiful. Amazing they appeared in less than 10 hours over night (after evening watering)!

Maybe your mulch contains mushroom compost, as much of it does - the byproduct of our local mushroom farms. I've often gotten mushrooms that look like those grown for market growing from my mulch. I would never eat them because I'm no expert and a chicken, but I'm pretty sure that's what mine have been.

Could be. More appeared for two more days. But the mulch went down a year ago last spring, so I would have expected them last year.

It doesn't really matter how long the mulch has been in place. It will continue to support fungal growth indefinitely or until it is fully broken down. The visible aspect of that - the mushrooms - will only appear when the conditions are right. Apparently last year they were not :-)

It was a drought year; gardengal, you're so smart!