I never said that every launch is staggered and always will be, but it could have to do with knowing that the SE would be launching in less than a calendar year.
Come to think of it, we got the 8/8+ and X as a staggered launch. The Xs and Xr staggered. Then the SE coming out less than a year after the 11 line. So if this launch isn't staggered it will be the first time since 2016 (7/7+) that Apple only has just one iPhone launch in a year. Even the year before that, the original SE launched between the 6s and the 7. So yeah, in the last 5 years Apple releasing all their phones for the year at the same time has happened only once.