How was Dende just thrown off the lookout by Popo and Dende just fell? Dende can fly

[Angelus]

This was when Super Buu got out of the ROSAT and turned everyone on the Lookout to candy. Popo threw Dende off

Dende could have even escaped faster if he flew downwards instead of just free falling

[Lionel]

It was probably a spur of the moment action. Super Buu was in the midst of transforming and devouring everyone so they needed to act immediately. We see following Freeza reverting to his final form how the little Namekian had momentarily been frozen in place from fear before setting out to heal Vegeta. Come the Buu arc, Dende may have matured a great deal but he was still probably in shock and horror at what was going on around him. It doesn't help that Buu had only just wiped out the remainder of the humans Dende was charged with watching over only 30 minutes prior.