Starter motor turns when power is connected.

[scole]

Just got done putting a fresh motor in my car, but i've run into a problem. I can connect ground and the fuse box and ecu power (everything but the power wire than runs directly to the starter), and everything works just fine. As soon as the starter wire is connected to power, the motor starts spinning. I cannot figure out why. Hopefully im missing something obvious, but does anyone know how this happens?