The fear of most companies being fake or unreal can make every miner with some coins sold at a price. Besides every miner have the right to sell their Bitcoin at any time for their own benefits because it is a business which they run every day, it is only for the beginners to doubt out of their novicism.
I don't think it's for their own benefit; it's pretty normal for companies to usually only sell their BTC to cover bills and maintenance costs, and I doubt they don't sell all of the BTC they mined.
At least they can survive mining while the remaining BTC is held for future benefits, which is essential if you own a mining farm; you benefit more from selling BTC at a much higher price because they already know after block halving, supply and demand always have a huge impact on the BTC price.