I submitted a paper in a springer journal. After just two days of submission status was under review. After another two days status changed to required reviews completed and your days have passed status is still required reviews completed. Why the process is so quick? How can the reviews be completed in just two days? What would happen next?
With a ton of context I'd be guessing; without any it's worse than that. Some subjects are no doubt much easier to set up reviews for. Some are much less competitive, needn't have the anonymity as protected, aren't technical. In many subjects your contribution to the field can be quite meaningful without necessarily being disprovable--so the burden on the peer reviewers is more like that on any critic, and less like that of a judge.
When I worked as an editor at a journal considered prestigious in American Studies and Western American Culture/history/writing and that sort of thing--before it went under the next year--my handling-time on unsolicited submissions fell along a bimodal distribution. I was as likely to hammer it out and handle it in the hour as I was to file it away for ... next ... lifetime? Least likely was that I'd get it done in some reasonable amount of time, like two weeks.
All in all I'd guess your piece was "easy" for them. That means one thing or the opposite. To put it kindly (and probably true): not a fit for that journal. OR, definitely a fit, and easy to find reviewers for because of that.