It probably has to do with the issues being report in this Multi-city pricing thread. The big 3 in the USA changed their pricing schemes - independently all at the same time, of course - for itineraries involving several cities on the same trip "in order to protect those travelers flying from those cities."

I recently booked a trip from LAX-PIT-PHL-YYZ-LAX that was pricing well over US $2,000 on both UA and AA. When I broke up into segments, the end cost came down to about $800 for a three segment trip, booked on AA both for convenience of the non-stops and a substantially lower price than UA.

You may need to break your trip in order to find a reasonable price.