you were likely going too heavy on your
shoulder workout. when you go too heavy, you tend to arch your back severely and shrug hard to allow your traps and lats to shoulder some of the weight.
pardon the term.
when you use free weights, dont think about "getting the weight up". that is a particular demand and your body will activate every muscle going in order to complete the instruction.
think instead of "straightening the leg", "bending the arm", "standing up", "raising the elbows", etc. this will activate ONLY the required
muscle fibres. you will, however, likely have to drop the weight.