Here's a "story" of the B tail (or at least one version of it)
Anyway, it may be because the descendants of the B had curly tails. I read this somewhere, "…Basenjis originate from pariah dogs, having characteristics particular to this group of canids. Pariahs have the tightly curled tail and a once year oestrus cycle, rather than the more common bi-annual oestrus cycle of the domestic dog…"
Interesting, cause I had heard just the opposite that the tight tails didn't get caught in the bush…
If you think about, our dog's tail is so tight it can't uncurl, it would easily get snagged and would not / can't uncurl. A regular basenji's tail can go limb and not get tangled. When they run it even straightens out behind them. Our tri's does not.
I have a girl that has a triple curl so it is curled pretty tightly but it easily relaxes and unfurls. I know that some people have dogs where it hurts to uncurl their tails but I do not seen in my own dogs a correlation between tightness of curls and whether it unfurls easily. My dogs do have permanent kinks in their tails but there would not prevent them from getting unsnagged.