there is a float in the base to detect leaks. there has been a leak and the float turns the pump on till the water goes
you either need to pull it out and very carefully and slowly tilt the unit back to about 45 degrees and allow the water to leak out onto the floor. dont do it quickly in case the water gets on electrics
take the side off and mop it out
or leave it 5 days and see if it evaporates
if it has a leak it will happen again but they can have bad days and flood
Dave