The reasons for market movements are always unclear.

I don't see any tax consequences whatsoever to making stake invisible. Moving coins from one wallet to another is not a taxable event in any tax system I know about.

The market can absorb his share in a little over a day

That is highly doubtful. Most of the volume you see on exchanges is day trading or arbitrage or even in many cases wash trading to inflate volume statistics (the latter is pretty well documented by now).

In order for the market to "absorb" selling 70 million Steem in a day there would have to be new buyers for 70 million Steem (roughly 25 million USD) on the same day. That's very unlikely.