Ok, it's time for everyone's favorite game show, Guess Cav's War Plans.
Possibility 1:
Cav decided to poach an available worker. This was their entire goal.
Why I find this unlikely:
a) They used 2 muskets, not 1.
b) The worker survived.
i) But, they might have thought they could actually survive for a turn (Clearly, a foolish decision)
Possibility 2:
Something is planned for next turn, but they tipped their hand because they wanted that worker. Much smarter, I think, would have been to wait a turn, let MS cut that road (would we have killed the warrior if cav hadn't declared war first???), then be ready to strike.
One if by land, two if by sea...