Small water drip during power outage...cannot understand the cause

Maybe somebody can explain this plumbing mystery to me.

Been in this home for 12 years now.

When the electricity power goes out and so the water pump doesn’t run, I close the blue valve which is the water line running forward of the pump and is the main water line into the home.

I then open the bypass water line- the red valve- which runs into the home directly from the municipality without need for electrical pumping…pressure isn’t great but it’s usually enough for a short period.

(see pic fyi)

Last evening the electricity went out.  As I have so often done, I closed the blue valve, opened the red valve…and went back inside the home.

I discovered the in the master bathroom shower, water was dripping from the shower head. It would not stop dripping.

Of course the faucets/valves were off/closed; and I thought maybe some sort of pressurization differential…there were no water anomalies in any of the other showers or any other point inside the house.

It continued to drip until the power was restored and I closed the red valve and re-opened the blue valve.

This has never happened before…anybody venture to explain why this happened?

Thank you…..

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