This afternoon, I noticed that some toadstools had appeared; this has not occured with other seeds that I started on the same day.

The seeds have been kept a steady 20 C, the tray wrapped in cling film (pireced for ventilation) and checked regularly for dryness.
At the risk of sounding thick, I've no idea if this is a common occurence with organic compost or if something else is the root cause.
Any help would be appreciated.