Bet it's the caliper dragging a bit.
Had the same problem on mine a few times. Well I have had it since 1999

. I normally strip the caliper and clean the piston up with fine wire wool and wd40. Give it a dam good clean, smear with new fluid and refit. Seems to work well enough for a good few years at a time. Also make sure the sliders/pins are free moving. If you grease these use rubber grease as petroleum based grease will ruin the rubbers over time.
Obviously jack it up first to see/feel how easy the wheel spins. Mine have only dragged a little and known to get really hot. I guess as they get hot they have dragged a little more , creating more heat and so on. Burnt me dam finger sticking it though the wheel onto the disc once

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As I understand it, it's a very common problem depending on the make of the caliper (think mine are the asin one's)