Overflows when draining. But only for the first few seconds

kenmore 110. We’ve had the washer in our home for way over 10 years.

What is happening is that when the water drains the initial push of water into the drain pipe overflows it. After the first few seconds it stops doing it. Washer is next to an outside wall with the cleanout just outside. We have verified that the drain pipe and swewer pipe is clear (again just during the initial draining it overflows.

It’s like the drain pipe needs to be primed or something. Once flowing its ok.