Shrey..
maybe because of your finger or the way you covered camera, telephoto lens is detecting the dark as your finger is already there, that's why it switched to main camera again, however be in white/light area and restart the camera and switch to telephoto lense and then put the finger there for about 5secs you'll see dark and camera will switch to main lense again..... that's the camera management :)

yes. now only I realize that thing. thanks for your info. it's so much helpful, and u said na there is lot of videos there in YouTube. I never see them before or I'm not saying after seeing those video's.. I just wonder myself to see that thing. after see this then only I saw some video in YouTube. but your theory is correct thanks lot.