One possibility for sure.

Another is there was a flight cancellation around yours or some type of connection issue with inbound passengers they knew about and you didn't.

A third is related to the rumors flying about CX cutting back on partner award space, but that's pure speculation. My sole anecdote is a friend frequently pays cash for what I think are AA award tickets on CX to/from LAX - someone or some company sells them to him, and he doesn't accrue miles on them. He told me the situation has gotten incrementally harder, especially if trying to "buy" two AA award tickets on the same flight.

CX is also just packed to the gills these days. More paid customers.