It's common on older trucks for the ground connection on dual filament bulb lamps to be bad, so the current from (say) the lo beam will hit the common point, and backtrack thru the hi beam filament to find a ground elsewhere (say in the other headlight). Happens on tail/brake lights all the time. Many of these lamps didn't have a real ground wire, they relied on the body of the lamp being grounded thru the bolts to the body.