Why this sudden change. Did all who wanted BTC for halving already sold them in past half year?
This. Everyone who doesnt really believe in Monero got out already when btc rose.
The remaining holders actually want Monero. (And most of us are probably well diversified between monero and btc and thus dont really need to sell one for the other).
Not necessarily true, and in fact, quite possibly the opposite.
Those who understand Monero's long-term potential would want to acquire Monero at cheap prices - a feat that would invariably involve temporarily jumping on a rising BTC train. I think that it's a bastion of human nature that people want good stuff as cheaply as possible... and Monero is clearly good stuff.
All that remains is the strategy employed. For some, there are no moral or legal boundaries. For others, it's a case of careful study of the trading variables and acting accordingly.
Believing in Monero, however, comes easily. It's the belief in where the charts will go tis the biggest headache.