1997 Chevy Express Starter

Electrical problem
1997 Chevy Express V8 Two Wheel Drive Automatic

Old starter was bad so I bought a new one. I followed the directions and got everything hooked back up correctly. Went to put negative battery cable back on and van tried to start by itself, ignition is off and keys are in my pocket. So I disconnected starter again and used a test light to make sure that there was not an ignition problem. Big wire is hot, little wire in not and I was told that that indicates that the ignition switch is fine and it had to be the solenoid, so I exchanged starter and the same thing is happening with the new one. Could someone please explain why this keeps happening and what a possible remedy could be. Thank you.