Does anyone know what's causing this and how to fix it?

 

I am guessing a leak from the hose?

Otherwise do you notice you are pushing coolant into the overflow after boosting?
To put your mind at ease, I would definitely burp the radiator and check if bubbles keep popping. So you can catch a problem early on if it's a blown head gasket.

 

looks like its time to replace your hoses. Get new clamps while you are at it.

 

There nothing in the overflow after boosting

 

I'm still new to Evo world, but in my Honda days, this was indicative of excess pressure in the system due to a failing head gasket. The compression would push coolant out the reservoir or the weakest clamp/hose.

 

The cap is suppose to be the weak point in the system. It could be stuck, but this hose is toast regardless. The hoses are part of the maintenance schedule. I'm guessing these are overdue.

 

Check if the metal neck has corrosion or deposits on its exterior. If it does then a new hose will also leak. Clean it up to a smooth metal surface.

I'm not sure what you mean by " There nothing in the overflow after boosting". If you mean the reservoir is empty, that's not right.

 

Like others have suggested, just check the hose and replace it if necessary.

I would, just for peace of mind, burp the radiator and make sure there is no head gasket problem.
I had a similar issue and diagnosed a blown head gasket very early. Replaced it and all was good to go.