According to this article, it was owned by a contract company named Hawkins & Powers. They own several other C-130As for fire-fighting services.

Simultaneous wing failure is extremely rare. The reports seem to conflict, but they were saying a couple days ago that the failure happened after they dropped their load, not before. Dropping about 3000 gallons of slurry would lighten the plane's load by about 20,000 to 25,000 pounds, meaning wing failure should be less likely due to the reduced stress.

However, every plane has an airspeed called maneuvering speed, which is the maximum speed you can fly without causing structural damage if hit by a heavy gust (such as a strong updraft) or by applying a full over control deflection. A plane flying at maneuvering speed will stall before exceeding the design limit factor, so the plane won't be overstressed. Here's an interesting tidbit - maneuvering speed decreases as the plane's weight decreases, meaning that as the C-130A dropped its load, a previously safe speed might've suddenly become higher than the new, lower maneuvering speed. He might've been flying too fast for the lighter weight and overstressed the plane in a strong updraft.

There's another factor that could come into play, especially for a 46 year old plane that saw hard military service. Every plane has a design limit factor (say 3.2 Gs) that it can pull without any structural damage or permanent deformation. Above that design limit, planes are designed with a 50% safety factor before anything begins to break. Therefore, a plane designed to 3.2 Gs won't start coming apart until it pulls 4.8 Gs, the ultimate load limit. However, pulling more than the design limit can lead to deformation that weakens the structure so that it won't be able to pull the ultimate limit in the future. That means that the plane might come apart at a lower acceleration, say 4.5 Gs instead of 4.8. The deformation would only get worse every time it exceeded the design limit, making the plane weaker.

Over the plane's 46 year career, it's pretty likely that it was overloaded from time to time and probably exceeded the design limit at least once. If that's true, then the plane was weaker than expected, making it more vulnerable to overstress related failures.