Ukraine and its supporters worry about losing control of the narrative

The attack on Kerch bridge is obviously not a spur of the moment thing, it likey took months to plan out. I suspect that the bigger picture here is that these attacks were supposed to happen in conjunction with a big Ukrainian breakthrough during the offensive, and then Blinken was going to put pressure on Russia's allies in the upcoming summit in Jeddah to force them to accept US position.

Looking from that perspective we can see what the probable US plan for ending the war was. Have a big victory on the battlefield, and combine it with attacks on Moscow, oil shipping, and cut off the bridge to Crimea. This would've been sold as Russia getting routed on all fronts, and then they were hoping that this would be enough to force Russia's allies to start having doubts which could be exploited in Jeddah. Of course, the reality is that the key component of this strategy failed utterly. Now, Blinken will go to Jeddah and likely face a reverse situation of what he hoped for where everyone will tell him that US needs to start accepting reality and negotiate with Russia.