Anti Lock shows up on 99 Galant

Before I had a flat tire the Anti-Lock wouldn't appear on my dash kit :). Yesterday, I had a flat tire in my rear driver side. Today, I removed the flat tire and I install the spare tire. However, when I turn on my car, the Anti (orange) Lock would appear. Everything seems to be fine. Could someone explain to me why my Anti Lock keeps showing? thanks

It will come on from either a fault in the system or a measurement of RPM difference from side to side of the car, especially so when a spare is on the car.