Well, looking up the etymology for "crawl" (emphasis mine):
crawl (v.) 1200, creulen, from a Scandinavian source, perhaps Old Norse krafla "to claw (one's way)," Danish kravle, from the same root as crab (n.1). If there was an Old English *craflian, it has not been recorded.
Then, the definition of "clawing one's way":
claw your way (somewhere): to move forwards with difficulty, especially by using stiff curved fingers to remove the things that are in your way
This would likely meet the idea that a "dungeon crawl", while initially thought to suggest a snail's pace, might actually mean "fighting your way" through a dungeon.
I am having a little issue finding any hard reference to the original application of the word to the series, but until then, this is the best I can come up with