He was under condsiderable pressure before ukraine kicked off, they were close to having enough letters of no confidence, and plenty of back bench tories were worried about the effect he was having on their chances. Several of them said they would wait to see ho the Sue Grey report panned out before deciding.
A pretty terrible local elections result will have done nothing to allay those fears.
By trying to drag Starmer into this as well, they're obviously trying to lessen the impact on Johnson. Assuming Starmer is not fined, this just gives additional ammo to those who want Johnson out if there are further things revealed by the Sue Grey report.