It depends on what is earthed (grounded) and what isn't. PC's usually have an earthed case, but many items don't - so when you plug the item into the PC it grounds via the PC. When it's not connected to ground via the PC, the mains isolation components cause a slight leakage current to make the casing slightly live - this isn't dangerous, but can be disconcerting!.
Another possibility is that it's YOU! that's charged up, with static electricity, and it's discharging from your body to ground via the PC. This could explain why you've only noticed it recently, because you're wearing different clothes?.