Allegiant is a tourist airline and has some really good software that does an excellent job of almost completely selling out every flight.
It adjusts prices up and down depending on supply and demand so it is possible to see prices drop after they are released (they can also increase dramatically too).
The following is my own observations of the industry so feel free to ignore my ramblings
An example: a flight that is filling more quickly than predicted by the software will move prices upwards since the demand is higher and the software is predicting fast sellouts (people are willing to pay more for a better flight and the airlines know this).
Conversely a flight that is not selling at the predicted rate at a certain price, will cause the system to lower the price of the flights to stimulate sales (airlines also know that even poor flights will sell if the pricing is right).
This is also why you will sometimes see a huge disparity between a flight going to Florida and the returning leg of the flight back to say North Dakota.
A flight on say a Friday going to Florida may have a high price (tourists wanting to start there vacation early and get the most time as possible) while the return leg of that same flight on the same day (say back to Grand Forks, ND) may have a much lower price since demand is much lower and the flight is harder to fill (who wants to come home on a Friday when you don't have to go back to work until Monday). Works the other way around also: a Monday return flight may command a higher price since you can stay the whole weekend and take 1 extra day off work or maybe Monday is a holiday. But who wants to wait until Monday to start there vacation and miss all those extra vacation days. If the pricing system is doing it's job properly both legs of the flight should sell out. The more desirable flights will bring a higher price and the less desirable flights will be a bargain since the plane has to fly both ways anyway.
This is why most airlines have the most expensive flights on the weekends (for tourists) and Monday and Friday (for business travel) and the cheapest flights can be had Tuesday, Wednesday.
If you are stuck on a tight schedule you will likely pay more for your flights even if you book as soon as flights are released. (Good flights generally command a premium price)
If you have a lot of flexibility and can take advantage of the lower demand/fare flights then you may be able to get a great bargain but could have a vacation that starts and ends mid week