If you are also a Twitter user and are happy that you have so many followers, then this news can shock you, though you do not have to worry too much because this is not due to any bug. This is because Twitter It is cleaning itself from the accounts which it has recently blocked.
In fact, in the last 2 months, Twitter has blocked millions of Twitter accounts that are running fraudulently and the rumours were being spread through those accounts. Now Twitter has taken the decision that the accounts which are blocked will be removed from the other users' Followers list. If you understand with the example, suppose that A's account has followed you on Twitter and the account is blocked by Twitter, now it will be removed from your list of followers and thus your number of followers can be reduced.
On his decision, Twitter has said that many of these users may have trouble with this move, but to maintain transparency and trust, it is necessary to remove the fake people from the platform. Actually a few days ago Twitter asked its users to change their account verification and password, after which many users did not verify or change the password. After that, it has decided to suspend and block them.