Most of the time for inexperience investors, they could take this as negative and become nervous and suddenly push the sell button. And then this whales are very happy scooping those BTC from noobs.
No need to panic with those sell notifications to xchanges though. Only those paranoid are those affected. And you don't need to hate this alerts, they are helpful on the other way around.
But yeah, in any case, we can speculate what that "unknown wallet" is.
That unknown wallet is refered as not exchange or known company, mostly are individual traders.