Due to the wear of something in the column shaft, It's causing a short.. ( of course it is Huh, That made a lot of sence) Mine does the same thing but only when I pull back,(UP) on the turn signal lever.. I can see the sparks cross from the Turn signal to the Steering wheel adaptor.. I use it this way.. Instead of having a "Horn button" I pull up on the turn signals and it honks. Makes me imagine it's a newer old truck.
If you would pull it apart, you could probably see where it is worn by the shinpieces.
RON