Installed new mandrel on my wizard AYP7143A99 riding mower. When I put it all back together, now the blades are ingaged all the time. Is belt routed wrong? I can't figure out what's causing this.

Suggest you have forgotten to re-connect the blade brake. This is supposed to stop the blades rotating when they're not supposed to. They are rotating when dis-engageddue to belt drag, which the brake is supposed to prevent.