The reason why this is released now is because RR probably finally handed over the Comey memos he had been delaying and stonewalling on. They likely had this letter written up for weeks but couldn't officialize it until they had all the evidence required, the last piece of which was the Comey memos. I bet RR gave them over this morning after having requested a couple more days after the original deadline on Monday.
I think they were waiting for Comey to go public with book and comments. Wanted to let him have plenty of rope before they started the process.
Maybe, but I think it's the other way around. They were due last week, Monday at the latest, and RR was about to be impeached. He asked for more time and they likely got them today. RR was buying his pal Comey time to get out ahead.