Southwest effect? It may or may not be fully applicable to your city pairs, but Southwest's $198 coast-to-coast R/T sale ended January 16. All the other airlines matching fares also ended that day.

As an example, through 1/16, LAS-BTV was $213 R/T (+ taxes) on a 7 day advance on UA and others -- and WN doesn't even fly there. Today, it is over $700 on a 21 day advance. Ouch.