Something to do with the sustain and una corda mechanism? Humidity and drastic temperature changes in the house?
Yeah, you could always take off the lyre and disconnect the pedals and see if the key still sticks? Where are the techies? I heard it could be a screw too tight or something in the action doesnt function properly.
i have the same issue! except mines an upright samick. the middle D and an A stick down (actually there's a lot more that do) when the pedal is down. it bugs me all the time because I use them the most. I got some guy in to look at them, and he couldn't really do anything. he just sanded down bits that were rubbing against each other, but that didn't help much :/