Travelers on 3 way switch are hot
The usual reason is that the installer is moving the wires to the same position on the new switch, rather than looking at the screws' function -- because every 3-way has their screws in different places and the installer did not realize the screw colors are the important part.  That's the usual reason.  

This is one reason I make a point to positively identify the two travelers and mark them with yellow electrical tape.   Yellow wires on yellow screws, there ya go.  In a cabinet with two unrelated 3-ways working in close proximity, I use blue, purple or green for the second set of travelers.  They sell a 5-pack of 5 colors of electrical tape for about $5 in most stores.