Maybe I’m reading way too much into this scene but it’s what I DO as a writer
But how they landed after falling from DEI, from a metaphorical AND literal standpoint, is really interesting to think about:
Perry, obviously, lands on his feet. He’s a trained secret agent, and right now he’s in full blown agent mode. He can’t afford to falter, because he not only has to protect himself from the dangers in this universe, but he has to protect his boys too. His emotional state is likely a whirlwind in this moment, but he’s had years of training to push it aside in times of imminent danger.
Ferb, interestingly, is sitting upright. He’s still on the ground, because he’s had the rug pulled out from under his feet in learning about Perry, but he’s stable. We see through his LACK of reaction to Perry that, while he’s stunned, he knows that this is not the time to react. They are in a dangerous situation, and he knows showing emotion will make him less aware of his surroundings (Note how once they’re in a safe-ish space, at their dopplegangers house, he allows himself to feel a little more). So while he’s still on the ground in shock, he’s much more prepared to jump up and move.
Phineas, of course, is the lowest to the pavement. He’s a very vocal character, so we don’t need to guess how he’s feeling - he, like Ferb, has had the rug pulled from under him, but he’s fallen face down because his emotions are getting the best of him. His only quick moments of rational thought in the prior fight sequence are in tandem with Ferb, who as we see a little later behind the dumpster is the sole center of stability in Phineas’ world right now. He’s in a very vulnerable state in this moment, face down on the ground, angry and upset, but we also see that he’s centered between Ferb and Perry - two of the most important people in his life, and also the two people who pick him back up (emotionally AND physically) throughout the rest of the movie.
Anyway thank you for coming to my critical analysis of PnF:ATSD.