It's sometimes due to inadvertently telling the Solo to move immediately after landing, such as by holding the left stick anywhere other than straight down and the right stick anywhere other than center. If you land and then (before it knows that it has landed) tell Solo to move laterally (right stick) or rotate (left stick), it'll try to obey you, and flip over.
So we need to know: what exactly were you doing when you were "powering down"? Powering down the Solo, or the controller, or your tablet, or by "powering down" do you mean something other than actually shutting down power? Had the props already stopped spinning? What positions were the sticks in immediately before the flip, and during the flip? Were you landing by manual descent or by using Solo's automatic landing procedure? Knowing the answers to these things can rule out some possible causes of flipping.