Mining Monero isn't even that profitable, I don't understand why so many are mining and dumping. Surely there is another significant cause for the selling pressure
It is the most profitable algorithm to mine on a CPU, therefore it makes little sense to mine anything else on a CPU, whether or not you want the coin. For example, you see on the BBR thread knowledgable people explaining that you should mine XMR on your CPU even if you are mining BBR on your GPUs.
There is no "selling pressure" beyond what has always been there, with 20K coins being mined every day that need to find a home. In fact botnets have likely always been there. What has changed is that demand has dropped from about 80 BTC/day to about 50 BTC/day. That's still quite a lot!