Originally Posted by
Bukhara
Circle Trip Minimum Fare Checks, HIF Checks and all that stuff are only used on 'Normal Fares' (ie, Full fares that are interlinable between carriers. Normally quite expensive). Special Fares (still often expensive, but restricted to specific carriers such as J2BA or YBMI) are not subject to these checks.
Chances are that you had a BA fare LON-CHI combined end-on-end with a CHI-SEA AA fare.
Thanks! That explains it as this was an I-class fare. It was just a BA LON-CHI (and CHI-LON) fare though, there was no CHI-SEA AA fare involved. So apparently the CHI-LON fare allowed routing via SEA on AA.