DD just started doing this & I'm not sure why. I'm thinking it's because she is so nosy and has to know what's going on around her (even if nothing is and we are in a dark room). She will take a few sucks and then sit up and then take a few sucks and repeat. I end up getting frustrated and stop feeding her and then she's still hungry.
Re: Sitting up while bfing?
I sit in a chair with DD on my lap facing me. That way she can look around and latch/unlatch as she pleases and I can bump :P
ETA: This is only at home. I don't do this pose in public!