They appeared because spores from some kind of mushroom landed in the compost when conditions were right for the mushrooms to grow. You may have had more woody material this time. They're just a part of the composting process.

The compost is safe to use on your garden. Even if the mushrooms are dangerous (they probably are not safe to eat which is the general rule about mushrooms unless you are an expert or get them at the grocery) it is your veggies you plan to eat and not the compost I hope.

Mushrooms/toadstools growing in compost bins is quite common and perfectly normal but as a search here will show, it is a common question often asked by those new to the composting process. ;) If you want more assurances that it is nothing to worry about, just do a search using the title of your post for many previous discussion about it.

Happy composting!

Dave

let em grow they will help decompose all the stuff you put in it.