This is usually due to filter cycles being too long, the pump operating for long periods of time will start transferring heat from the pump into the water heating the pool. This is more likely to happen in summer with warmer weather.
Filtering of 4-8 hours a day is sufficient for most pools.
Changing Filter cycles Balboa TP600 - see here
Changing Filter Cycles on the Davey SP601, please see page 6 of the manual below.