How often for prior routes change?

I am on a flight from ord to iad later today.

Last night I was tracking inbound flight route and it was quite complicated with three legs beginning at 6 AM. This morning I rechecked to make sure prior flights on time and saw the prior flights had changed.

Questions for UA folks: 1. Are these flights planned manually or is there a computer program that automatically routes planes? 2. How often do changes occur on day of flight and why? 3. Does the same flight every day have the same prior routes or is it not fixed?

Thanks