The reason for that is that the seller is maximising their trade by pitching the order at the optimal liquidity level.

If it's a half decent exchange (not Cryptorush) the trading engine will automatically feed them the highest bids, even though they've placed their order at a lower level. If someone's cashing out a large amount that's the only way to do it because if you let a large sell order sit on the book at above the top bid, all you'll do is push the bids down out of reach and destroy all the liquidity that built up during your 'waiting time'.

All this behaviour is totally consistent with a whale cashing out.

By the way, that's also sometimes the reason why you see sellwalls being pulled as the price is rising. It isn't always because people want to "hold". They're getting ready for a cash out by encouraging the bid liquidity to accumulate enough to absorb the dump they're about to execute.