Were you able to tell if the engines or crew transports were northbound or southbound? I suppose in Kingman they could have been east or west bound as well, but in Kingman, in between the two junctions for U.S. 93 it is hard to tell.
The reason I ask these questions, which you might not be able to answer, is that I have a theory. And no matter how much training is conducted not to do this, they might have been on their District frequency in the direct or simplex mode, even though they were likely quite some distance from their home unit. When I was in the U.S. Forest Service and traveling about for fires and investigations I would, from time to time, hear strike teams of crews and engines using their home unit frequencies hundreds and thousands of miles away. This is likely what you heard. Some units might have had their tone box set to Tone 15 (162.2) and some did not. This would indicate that the last time they used their radios on their District's repeater net, they were using Tone 15. I can't hazard a guess as to why some did not have a tone on their transmissions.
I do some poking around in my notebooks of info and see if, by chance, 171.1125 is still a tac frequency in some BLM State Offices or if I can find this frequency used by a particular BLM District.
By the way, six crew transports don't usually travel without a strike team leader who would be in a pickup. Six vehicles, 5 engines and one strike team leader in a pickup would make sense as it would be 6 vehicles total.
In the future, if you make an observation of fire vehicles, it would be helpful if you kept the questions I asked here in mind when you make the observation.