Ohh, sorry I thought you were just asking why anyone would be expecting a spring event to begin with.
Anyway, personally I'm not positive that the new iPad Pro will be released in the spring. There's a quote from one of Gurman's newsletters that mentions the new iPad Pros:
There’s really nothing else left of substance in the road map that would be ready for 2021. Instead, look for a larger Apple Silicon iMac, new Mac mini, new iPhone SE and new iPad Pro after 2022 gets underway.
That implies an earlier-than-later release in the year to me, but he's noticeably careful not to peg down an exact timeframe yet. I think an 18-month release cycle would probably be the most likely for the Pros going forward, but I could see 12-month updates here and there based on silicon timeframes.
I do think the next Pro will have to have the M2, and I expect the M2 to debut first in the MBA. Maybe there'll be an event for the MBA and iPad Pro to both get the M2 at the same time? I could see a ~May event where that happens. Or maybe the MBA gets M2 in ~May and then it trickles down to the iPad Pro in ~October... Hmmm... Many possibilities, and it's all exciting.