Certainly more risk of dripping corrosive stuff into your headset in a trainer situation than on the road, so that probably played a part. A front end loose enough for the fork to contact the headtube isn't normal, though...and I'm not sure a corroded but otherwise intact set of bearings would cause it. Certainly it would be something you'd notice quickly out on the road, so I'm guessing things were fine before you went indoors for the winter and something loosened up during your trainer sessions?

The torque maximums you note are for two separate parts...the preload nut is set to 2.5Nm first, then you tighten the stem clamp bolt to 5.2Nm.

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Carl Matson