He could have waited out the year to see what, if anything, would happen that might make the Tories' inevitable defeat less absolute, but he decided the kamikaze option was the right one for whatever reason. Possibly he's looking forward to an early retirement in the US.
My biggest concern for some time now has been a Labour government without an effective opposition. If the polling is accurate then complacency will be the defining characteristic of Starmer's time in No 10.
Lastly I'm not sure if the date is supposed to be some sort of stunt because muh independence but it certainly seems a touch Boris.