It is an expansion tank, not an overflow hose on your boiler. This happens every time engine gets up to operating temperature. Coolant is not compressible, but expands a lot. If you look into expansion tank you can see how level fluctuate between hot and cold.

This nipple is after the spring and cap, the other end of this hose it drains into unpressurised expansion tank that is open to the air. How any pressure can build in it? Doesn’t absence of any clamp on the hose gives any clue?