I actually made a thread about that same thing happening to me in the "coil builds" forum.
It was deduced that it happens when the wire was not annealed prior to wrapping. Since I went back to annealing before wrapping it hasn't happened.
Notice how the wire dances / bends when you anneal it. Well that same thing is happening when you fire it I guess
I use a little torch but a bic lighter works just fine. I just run it along the length of the wire, wrap the coil, cut the wire and mount it into the RBA. Then fire, stop firing, run my mandrel thru a few times (to alleviate hot legs) fire again to observe the hot legs magically disappeared then wick and vape like the freak I am
I run 29 ga.