It was likely from an ad, though without being on your computer I can't say that for certain. TPB doesn't purposely embed malicious content into it's webpages, so I'm guessing it wasn't something intentional. If MSE was doing a routine scan, it could have picked up something new, though I would tend to agree that it happened while you were browsing online.
It's also possible that whatever it was wasn't actually malicious, and you got a false positive. At any rate, I'm glad MSE is working out well for you
