Why can a Musaafir not Perform Qadhaa salaah behind a Muqeem?

Q: Salaah-503: Nurul Idhaa states that once the time for a salaah has expired, it will not be permissible for a Musaafir to fellow an Imaam who is a Muqeem. What is the reason for this?

Answer: The salaah of a Musaafir can be changed within the time for salaah If for example, he changed his intention to travel. He will also follow a Muqeem Imaam in four Rakaahs. However, once the time for a salaah has finished, his Salaah will be fixed at two Rakaahs even if he becomes a Muqeem. He will therefore not be permitted to follow a Muqeem in Qadhaa salaah since the Muqeem will be performing four Rakaahs and while the first Qa'dah of the Muqeem's will be not be Fardh (obligatory), the Qa'dah will be Fardh (obligatory) for the Musaafir.

Fatawa Rahimiyyah vol.2