Based on my past experience, 2 things could have happened...either you needed to bake them just a tad longer, or the batter was too moist. Depending on what recipe you used, you may have needed to add more flour. HTH!
Having the same problem...have no clue though. Didn't think a cupcake could be TOO moist. 
It has nothing to due with being underbaked or not enough flour. The reason I guessed you were using a doctored mix is that if you add in other acidic or sweet ingredients into a balanced recipe ( like a cake mix) you can create an out of balance end product which will draw moisture from the air ( sugar is hydoscopic, as are many acids like lactic, the acidin milk) .
Your cupcakes are getting slimy because you've pushed the recipe out of balance in some way. It won't really hurt anything, but don't let them sit around like that. The mosit environment is very bacteria friendly.
what about when they're baked from scratch?
I would guess it is the recipe you are using. The epicurious double chocolate cake recipe that everybody here uses will go a little out of whack and get sticky on top depending on the Ph of the cocoa you use.
I think I may have answered my own question... I store the cupackes in a covered cake stand, ( it is not air tight and the humidity in the air might be causing this). The next time I make the cupcakes I will store them in a airtight container and see if they still get sticky.
my cupcakes get like this too. should i put them in the fridge before they're fully cooled?
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