Hello, My name is XXXXXXX XXX I will do my best to help you today.

Your system is either freezing up due to low refrigerant charge or poor airflow (dirty filter, dirty coil, malfunctioning blower motor). If this was the case then your home would not be cooling. If your home still seems to be cooling you just have water in the pan then.....

Your drain line is plugged up causing the condensation to exit the coil drain pan and flow into this safety drain pan.

Or the coil drain pan has a crack in it

If the system was freezing up the house would get hot it would not hold a temperature not even 78 degrees on a 85 degree day.

Chances are the drain line is plugged. There is a 3/4" pvc line that comes off the coil drain pan and exits the home. If this drain is restricted or has poor gravity fall then the water will back up and overflow this coil drain pan causing the water to go into the safety overflow pan.

Does the safety pan have a drain on it that drips out over a window somewhere on the outside of the house?

It could be that the main drain is just partially restricted and it can't keep up so only some of the water goes into the safety pan.

If the safety pan doesn't have a drain off of it then it has to have a float switch installed on the pan which cuts the unit off if the safety pan fills with enough water to engage the float.

If not then if enough water gets into the pan it will flood the ceiling of the house. below.