1999 Silverado

I have a 99 Silverado with a 4.8l engine and automatic trans. When I went to crank it, it would not turn over. I took the starter off to get it tested and it tested good. As I was making the last battery connection after re-installing the starter, the engine started turning over; without a key in the ignition. Does anybody know what could be wrong? Thanks for your help.

You must have put the wires back on wrong on your starter solenoid- double check them and try to remember where they go- or you have crossed up the hot wire to the battery up against the solenoid post somehow... in which case you need to isolate them..

Thanks for the info. I will definitely look at the connections again.