Desperation made the fourth-down decisions easier
The first answer is simple, the Raiders were backed into a corner against the Chargers, Jon Gruden finally saw his playoff chances and his season slipping away, and needed to do something to try and change that. Las Vegas was getting no traction with their defense and desperately needed to win the game, so Gruden threw out his conservative leanings, until overtime at least, and decided to start rolling the dice.
Raider Nation might have been wondering where that tendency was all season, as there were several opportunities this season where Gruden opted for a field goal when chasing a lead, rather than going for it. The games against the New England Patriots and Tampa Bay Buccaneers stand out, but there were surely other chances that Gruden turned down.
What is particularly infuriating about Gruden’s insistence on being conservative is that the Raiders’ offense has been the strength of the team the entire season, and he consistently chose to rely on a terrible defense. The Raiders are still top-3 in the NFL in third-down percentage despite some recent struggles and have had great success on fourth down albeit in a much smaller sample size.
Gruden is an offensive-minded coach and has done a great job coaching up a great offense, outside of the red zone at least, but when push came to shove, he chose to pass the buck to his defense. Against the Chargers, however, aside from the disparate nature of the game, he had another weapon at his disposal that he did not have all season.