My vintage figures are all sticky after decades of being out of their packages. Might be cuz they've been in a big ol' wooden toybox with all of my other childhood figures. Maybe they react to the plastics from other figures too? Could just be time and air/heat/moisture exposure though, I s'pose.
I don't think there's anything you can do about it though, sadly. The front half of my 200x Skeletor's torso started yellowing a couple years after I got him, except for under his armour. There's a blue X pattern on his chest now. :P