I'm incredulous about it being easy/cheap to get 10% of a stake of a well functioning coin. If you can get 10% of a stake without buying and want to profit from it, the easiest way is not to give back the 10%, and sell the coins on the market.
These things are not mutually exclusive. The selling that you brought up to discussion could be one part of the plan. Actually I think the selling of the coins is going to be prominent part of most of the attacks. Let me remind that selling of a big stash usually does not happen instantenously when talking about a large stash like 10%. There is quite typically first a contract signed that there is going to be a sale and some time after that the coins actually change owners.
The period between signing the contract of the sale and the actual transfer of the coins is a perfect place to attack the coin where the seller has nothing to lose and in many cases quite a lot of to win by doing so. And there are situations where the timegap between those events is naturally quite long like months. A well functioning monetary system allows all kinds of transactions including ones where somebody can buy a big stash of coins in such a way for example in a situation where a whole bank/exchange business is up for a sale.
In PoS economy it is very risky to buy a big bank or exchange business from its previous owner. Think about a situation where the owner of a bank/exchange with big stash of customer coins in their possession has decided that he is more a risktaker entrepenour type of person instead of a person that runs an established mature boring business and he wants to cash out and start over from scratch and take new risks on some other competing emerging new kind of coin. Quite natural event that I guarantee is going to happen thousands of times.