It was indeed a snafu on Twitter's end on Friday when a large number of people who did not follow the @POTUS account nonetheless were treated to President Trump's first tweet from that account in their feeds. A spokesperson from Twitter initially denied that anything like this happened when asked by SFist, but in a series of tweets on Saturday CEO Jack Dorsey addressed what happened, which he says was a technical error and "it wasn't right, we own it, and we apologize. No excuses."
Dorsey says they believe they've corrected the problem, in which an estimated 560,000 Twitter users were made to follow @POTUS against their will they were either users who had unfollowed @POTUS but followed the account in the past, or they were new followers of the newly created @potus44 account for President Obama, who started following that account after noon Eastern Time on Friday. Twitter did a massive migration of followers starting at that point, essentially duplicating the follower base for both accounts, but the migration did not happen instantly and there appear to have been problems along the way.
Over the course of a couple of hours, the follower base for @POTUS appeared to rise dramatically, and this was because the 14 million followers who had been following the account under Obama's watch as of noon Friday were being added back. It currently has 14.2 million followers, while @potus44 has 14.5 million, and @realDonaldTrump has 21.5 million.
Obviously this was an unprecedented situation given the fact that Twitter was not really a thing back in 2009 when Obama took office, and we haven't before had a President so enamored with the platform as Trump.
Below, Dorsey's full explanation.