To put the cat among the pigeons

to say or do something that causes trouble or makes a lot of people very angry

When my brother’s company imposed a 15% pay cut on all employees because profits were low, lots of people left and others were furious; it really put the cat among the pigeons.

The idiom originally referred to the disturbance likely to be created by putting a cat inside a dovecote (or dove house). This disturbance is caused by the cat's tendency to hunt and kill the birds.