The Biden Campaign just figured out that being the underdog has the advantage of motivating your supporters to cast their ballots. If the election was leaning heavily in favor of you, a good number of would-be voters would blow off casting their ballots.
Biden has been polling double digits above Trump, but now they are warning the polling "might be wrong"!

Might be? Anyone who followed 2016's failed polling already knew that.
The press has been pushing the narrative that Biden is WAAAAAAY out ahead. I've been of the opinion for months that those reports only help Trump and hurt Biden with voters.
I think the poll numbers likely helped Biden raise money (would be harder if he was predicted to lose). Now that it's down to the last 2 weeks and a few days, they are flipping the script to make it look like the race is a toss-up in critical swing states.
Gaming the system with help from the polls and media. Whodathunkit?