My eleven and a half year old daughter has these small flaps of tissue lifting up behind her back teeth.. what would this be caused from?

Not sure: Sounds like gum tissue. Without seeing it am not sure. Make an appt with her dentist, especially if it is causing any concern or discomfort.Could just be normal and could be her 12 year molars getting ready to come in.

This is common: If she is 11+, her 12year molars should be coming in behind what used to be her last teeth. As the tooth errupts, there is a flap of tissue on the back edge that is common. As the tooth errupts more this usually goes away and cures itself. In the meantime, it is important for her to clean under it. If it is troublesome, or doesn't resolve, your dentist can remove it.

Operculum: Fancy name for the flap of gum tissue that was covering the complete tooth but as the tooth comes in the gum tissue receeds away. The very back portion is the last part to go away and can take a while. It can be sore but the best thing is to encourage them to chew solid foods back there and it will help the tissue to receed.

Coronal flap: As the 12 year molars erupt the overlying flap of tissue has to split and shrink back. Keep area clean and see your Dentist to make sure that everything is OK.