I wouldn't say they specifically pick up creators from Tapastic, they really do seem to have a lot of factors in mind. (unOrdinary came to mind. I believe they weren't on Tapastic either) Most artists would try to spread their comic on the most popular comic websites and hope to do good on one or another. Although:
Well, there you go, I think that's a pretty good reason you just threw out there. Tapastic is a small community and everyone around here knows how hard it is to reach that 1k subscribers. So for a comic to get that big on Tapastic, THERE MUCH BE a reason. The comic is probably pretty good and they stands at higher chance to be picked up by Webtoon if they came across it.
Most comic maker on Discover probably would tell you that they want to be featured. I could say the same for myself (although I'm not sure yet since becoming a featured artist is a lot of hard work for a full time student that has a irl part time job like me) I have seen MANY readers commented on MANY comics that they want this series to be featured, even though the quality imho is not up to bar with what Featured webtoon offer. (Don't take me wrong, personally there is some on the popular chart that totally deserve to be official, but many aren't like that)
Also this LOL.