They could be doing this to cover their operational cost and to scale, or they are forecasting another dump?
The miners can also be selling to buy AI infrastructures which is what few miners are doing now, but I can be wrong. The miners that sold the coins will be the that can give the main answer to this question.
210,000 bitcoin is not a joke for any day and could likely influenced the market especially when they have that mindset of dumping on the market
It is ~21000 BTC and not 210000 BTC. 5 figures not 6 figures.