There are various factors interacting together and resulting in final enclosure humidity level. Room humidity and temperature; enclosure heating (amount and type); and enclosure ventilation. A couple possibilities here are discussed below.

The hygrotherm is not working right (programing or sensor). If fogger is kept off during humidity spikes over set parameters (80F) then cross this one out, however if fogger comes on at readings over 80%, have unit checked out. Where is sensor located and is it being "wet" by fogger output? Any condensation on a humidity sensor will affect it's readings. Also, get a digital gauge and compare readings to hygrotherm's to ensure it's reading are accurate.

The room humidity is higher than 80% as BC discussed. Have you measured the room humidity? If so, remove fogger, and get room dehumidifier if possible. Coco substrate should only be slightly damp so it starts to clump in closed fist without dripping water out.

What do you use to heat enclosure at the present? Heat from infrared bulbs and ceramic heat emitters will dry air and help lower humidity. Good luck !