This is due to them having too much action on one side, and they need to limit the action. Usually it resets later on, but probably requires a manager intervention. They probably have a cap of like $50-100K for a major line. (remember, they only rent/license these live lines, they have to limit their exposure.)
On some line individually, I think you can only put $5K as max win amount too. I remember some games I had multiple to win $1K bets and was limited. But that wasn't their side limit, was an individual player limit.
Sometimes you can actually bet the "Maximum bet amount of this proposition down to $0. You might see it was $284 and you try to put a $300 bet. Then you bet $284, and the new Max on the proposition is now at $0.