Usually this is caused by a slow drain.
The water should drain quickly.
I assume you checked the tray on the bottom to ensure a dust build up is not slowing the drain
Did you flush the system with a 10% bleach solution. Crud likes to grow in drain tubes and it slows down the drain. You want to kill it and flush it out. Often a turkey baster works for this.

Now for the jiggery pokery, NOTE I have never tried this but have seen plenty of posts from others who swear by it.
Take a length of solid bare copper wire (14 gauge) coil it a couple times around the defrost heater and then run this down the drain tube an inch or so.
Cannot tell if you have a calrod (black ceramic) or glass heater. This fix may shorten the lifetime of the glass type heater.