The timing makes perfect sense. A certain set of Bernie supporters are determined to torpedo Biden any way that they can..
If the assault had actually happened, it seems odd that she never mentioned it during either of his runs as VP. But she didn't. She saved it up for now, after a lot Bernie support and Putin love.
To be fair, Christine Blasey Ford didn't bring her allegations against Brett Kavanaugh until it was clear he was on Trump's short list to be on the Supreme Court. Not when he became a judge or rose in the ranks, but when he became a candidate for the highest court.
That said, the more important point about Biden having been VP is that not only did the Obama campaign thoroughly vet him, the GOP had plenty of opportunity to dig through his life for "opposition research" and I can't imagine they'd have let any evidence of him as a serial sexual predator lie around unused. I mean, Obama's opposition flat-out made up a fake narrative about his birthplace, used his choice of church against him, and hammered away at the fact he had interacted with a former 60's radical (Bill Ayers) which is not really controversial when you consider they were both professors in Chicago. The opposition had to gin up these "controversies" about Obama because in truth, the man was a boy scout by comparison.
The easier path would have been to go after his VP pick, who'd had a long political career, yet for some reason not a single one of these supposed indiscretions came to light until 8 years after the last time he ran for office? Hmm...