Answer: Mushrooms grow because there is decaying organic matter at or below the soil surface. As long as the soil is moist, mushrooms will grow. Sometimes they will pop up where tree roots are decomposing and sometimes they will appear in wood mulch because the mulch is slowly decomposing. Without being able to pinpoint the exact source of decaying organic debris, I'd simply pull them up or rake them up and throw them in the trash. Try to do this before they fully open so you can stop them from developing spores which can turn into new mushrooms.
Best wishes with your garden!
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