Take a look at the entry for "どかどか" on デジタル大辞泉.
While it is true that "どかどか" is strongly associated with the "thud-thud" produced by footsteps, it is also used to evoke the idea of things being or moving in large quantities at a time.
Here are a few examples found on the web involving the precipitation of snow.
There is also a related term, "ドカ雪".
I think it's a fair bet the semantic extension from the "thud" sound to the idea of large quantities simply traces the reality of a loud sound being made by things in large quantities as they move or come into contact with some object.
A quick tangential aside -- "どかどか" can also describe the way a single individual walks or runs, and sometimes it has more to do with the manner (rushed, inelegant, discourteous, etc.) or how the speaker feels about it than the sound per se. A couple of examples:
The "どかどか" in this sentence is indicative of the rudeness and offense he felt of the trespassing.
This sentence recounts how the writer's cat Tamako took an insouciant stroll across her Mac computer. (I doubt that any loud sound resembling a thud was actually made in the process.)