Answer: I think it's more of a coincidence than anything else. Even if the potting soil is sterile when you buy it, as soon as you open the bag, it's exposed to all kinds of fungal spores that are always floating through the air. You are correct in your procedure - pick out the offenders and destroy them - and remember to wash your hands so you don't carry the spores from plant to plant. If you keep your soil a bit on the dry side, you should see fewer mushrooms.
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