Because if you look in their feeds they all talk about the same cryptos when they do post. They RT tweets for the coins they mentioned in their own tweets. And they RT giveaway tweets all replying with different addresses for claim. Usually 100 - 200 RT's per giveaway post.
It's just one guy shilling for hundreds of claims on giveaway posts and bombing inboxes with "buy my followers!".
It's not that huge of a deal. It's just annoying when he blows up your phone with the same DM over and over, and can be good to have a list of shills to check from when running twitter giveaways.