Rainbow Dash was going to give Scootaloo a flying lesson, but forgot all about it because she had something else to do. Scootaloo had a crash landing and Fluttershy came over to fix Scootaloo up. Rainbow Dash arrived too late and isn't game enough to apologise yet. She observes from a distance.
I know that Scootaloo looks up too RAinbow Dash and all... But... She's more like fluttershy, because; they both were/are teased about the fact that they couldn't fly, and they're both kind of... Errrr... Traumatised? By the teasing and bullying
Also, having Dashie hiding behind a tree makes me think that maybe she was afraid of not being good enough of a teacher. She then freaked out a bit and asked fluttershy to give scoots some tips instead of her. Feeling guilty, she decided to at least watch what would happen.