One important additional detail is that the Starship started shedding debris (both inside and outside) and tumbling slowly, right after SECO. A leak also seems to have developed at the same time - which would explain the gradually worsening tumble rates. Leaks are unfortunately difficult to distinguish from intentional venting, if you're unfamiliar with the venting scheme. However, it's more or less surely a leak(s), especially given that the same were not observed in previous flights.