Twitter’s jumping on the bandwagon of having videos automatically play while you scroll through your timeline; that is, if you use iOS app or Twitter on the web. The move to have Vines and other native Twitter videos play automatically is meant to keep users engaged on the platform. How effective that is we can’t tell at the moment. Those using iOS will have the clips go into full screen if you’re viewing the videos in landscape.
If you’re worried about this eating up data, you can opt to only let video autoplay while using Wi-Fi or just turn it all off.