It's usually incompetence
To paraphrase House, MD: It's never Evil, unless it is.
In all the fury of the past weeks about this issue, there have been countless accusations hurled at Twitter, about how this was a willful choice by White Dudes or something over at Twitter HQ, done in full knowledge of the consequences of this algorithm. None of that really adds up, though.
First of all, Twitter's red-headed stepchild (TweetDeck) doesn't have any of these issues, it uses a simple centering algorithm. In the case of all the 'damning evidence' tweets people have been sending around lately, all I could see on those was the white area in the center of those images. In the few cases that I am forced to use the burning trash fire that is the UI at twitter.com or the toxic waste spill that's the Android Twitter app, I'm always impressed by how much 'fancy' stuff they try to cram into both.
The sorting, filtering and tenderising of one's timeline (even when supposedly disabling this behaviour), the countless sponsored tweets and the countless UI and UX glitches that were the primary reason why I only use TweetDeck. In its barren, neglected simplicity, TweetDeck only does the bare minimum.
That all leads me to believe that the problem with Twitter is that as a company that's barely scraping by as a money-losing enterprise is not a beacon of technical elegance or competence, but more one of 'look at the shinies' while praying that they can draw in enough advertising revenue that month to keep the lights on at Twitter HQ. I doubt Twitter even has the budget to invest in Evil Schemes at this point.
Tl;dr: Shiny feature, not really tested, backfires nicely. Just another corporate day at Incompetence'R'Us :)