Of course it's all just backroom conjecture. Five Senate Democrats said publicly last week that it was time for articles to be passed to the senate, which has been uncommon so far with the impeachment proceedings. But nobody knows for sure why Pelosi signaled on Friday that she'd prepare to pass the articles over to the senate, whether it was because Senate Democrats started to publicly state they wanted her to, or if it was backroom requests, or if it was because McConnell hasn't budged since Pelosi first withheld them, or maybe even if the Democratic primary candidates from the senate want to use it as an opportunity for a high profile rebuke of Trump with the Iowa caucuses & New Hampshire primary approaching (Warren, Sanders, Biden, Klobuchar would all have an opportunity to be involved in the trial, an advantage they have over Buttigieg, Yang, and the other non-senate candidates). All we know is that a number of Democrats started to publicly ask for the articles to be moved to the senate, a handful of OpEds came out encouraging the same midweek, and by Friday, Pelosi signalled she'd be moving to that step this week.