Yes, it is because of NATO. They've been thinking about it for some time, but NATO tipped the scales.

Russian military logistics are train based. If Finland switches away from their rail gauge, it's safer from Russian attack, since Russia wouldn't be able to easily carry supplies farther inside Finland.

From what I know they can be rebuilt easily and Russia has tons of engineering units for this specific purpose. That's what they did in Ukraine.

I assume that changing the gauge also affects other related equipment such as signaling and such, which would make rebuilding much harder.