Why do my spark plug wires have suction when I pulled them out to change the plugs

 

Probably because the spark plug has a rubber seal that sits on the porcelain. When you pull the boot off, the seal creates a vacuum.

A very light coating of silicone grease on just the inside of the rubber boot should help it slide off and not stick to the plug. You don't want to get dielectric grease on the top metal post of the spark plug/spark plug wire. If you pull the plugs, you could put a light coat on the porcelain as an alternative.