PSS Vent Leaking

[noelex 77]

For the first time today I had some seawater leakage out of the top of the vent line for my PSS shaft seal.
It was only a small amount (perhaps 5 table spoons worth), but I have 2 questions
Why now?
What do I do to stop it happening again ?

The vent line (a simple tube no pressure feed) leads up to just below the cockpit floor about 1.1m above the waterline and 0.35m from the centerline.

The puzzle is it has never happened before (the engine and vent arrangement was changed about a year ago). I went for a 30 mile sail in light conditions and anchored in a new location. My theory is the reverse thrust from the propeller when setting the anchor forced the water up, but I always set my anchor the same way.

The boat has just been re-launched with new antifouling but nothing with the vent or stern tube was changed. The propeller a maxiprop was re-greased and one anode changed (same size and location)
The vent would be difficult to locate higher I am thinking of a simple water trap on the top
Any theories or solutions ?