At a High or Solemn Mass, several key texts (including the Creed) are recited by both the sacred ministers and the choir separately. Naturally, it takes the choir longer to sing a text than it takes for the priest to read it from the Missal.
Sometimes, the choir starts earlier than the ministers. This happens at the opening of the Mass, for example, when the choir sings the Introit and Kyrie some time before the ministers finally reach the altar and the Missal (which is where they will quietly read the Introit and recite the Kyrie).
At other times, like the Creed, the priest and choir begin together. In this case, the ministers finish and wait for the choir to catch up. For this reason, you will often see the ministers standing patiently at the altar before the choir reaches “descendit de cœlis, etc.”: they have already finished saying the whole Creed, and are waiting to genuflect/kneel at those words. They then sit (and put on their birettas) to wait for the choir to finish the Creed. As Tis_Bearself notes, the congregation is simply imitating the ministers.
I agree with you, Fets, that it feels wrong, and I think this is because you and I are following along with the choir rather than the ministers. The ministers are seated because they have finished the Creed; you and I wish to stand because we have not yet done so. If you keep an eye open at the beginning of Mass, you will see that some people follow the choir (listening to or reading the Introit), and others the ministers (kneeling and beating their breasts as they confess their sins, which is what the ministers are doing at the foot of the altar). The FSSP parish in my area explicitly states in their little handbook in the pews that one may follow either the ministers or the choir; in practice, though, the congregation follow the ministers, in the manner you describe.
I hope that this clarifies things.
