Her attorney had her take the polygraph.
This is not some conspiracy by the D's to wait until the conclusion of the Kavanaugh hearings then drop the big bombshell. It just took that long for Ford to decide she would allow her name to come forward - this isn't one of those TV moments from FOX TV where the accuser calls up the local precinct to report an incident and by morning of the next day it's front page news. She was vetted for truth by attorney's and WaPo for credibility. There wasn't some planning over two months and a half months - it should be noted, she called her congressman when she saw Kavanaugh's name as a possible choice by Donny dipshit.