This is speculation but grounded in a lot of knowledge and experience.

What AA did to you in Austin was probably because their DFW flight (the one you were originally on) was oversold. The ticket agent was motivated not in getting you to Philly but getting you off the DFW flight. If you would have stayed on the DFW flight and you would have either volunteered to change or have been involuntarily rerouted (denied boarding) you would have gotten money, etc.....

Next time you need to be a little more suspicious of an anxious ticket agent wanting to reroute you. If you asked her if the DFW flight was oversold, she would have told you the truth, and you could have at least gained something upfront from your reroute.