Every time my sump pump kicks on the water in my toilet bowls goes down.
Any guess what's going on?

A sump pump should not be connected in any way to the sanitary sewer system (toilet).
Could you be talking about a sewage ejector pump?

Are we talking sump or ejector pump? If it's a sump pulling a vacume on your drainage system then vent the sump pump just like any other fixture. Good luck. Tom