Twitters profitability has always been a contentious issue, they launched their first subscription service (Twitter Blue) last year. My assumption is that they are potentially trying to move towards a more subscription based revenue model. The push back against ads is only going to get stronger and maybe they are trying to get ahead of the curve.
On top of that with GDPR and all the inevitable legislation that's coming, having people "registered" ensures that they have accepted Ts&Cs and can be tracked legally for advertising purposes. Maybe they anticipate not being able to run (particularly) profitable ads for unregistered users.
Finally, with the new CEO he probably wants a quick win on metrics, this could push more registrations, good news to report to the board.