It is a classic mistake by greedy people. They do not have time to care about the privacy of their keys, they want only money as soon as possible. Most of them are participating in fake airdrops: Give us your private key to verify the address and then we will send you the airdrop.. 
How can you give private keys to verify addresses? i wonder how people fall for this foolish trick, i guess we have too many newbies who don't learn how crypto wallets works, that's why they are easily fooled, any airdrops or bounties that ask for private keys are scam