According to BA Source it was late departing O'Hare due weather and planned Manchester as destination due crew hours limitation.
Presumably flight time to Manchester was longer than expected so went to Shannon before crew ran out of hours.
Based on many years SLF experience, transatlantic flights that overfly Manchester go nowhere near Shannon, and vice versa. Perhaps they went into Shannon because they had a more southerly routing than expected.
The appearance of a 773 on the LHR-MAN run may be linked to the BA294 issue, but it could be a coincidence caused by non-availability of an A320-series. It would presumably be much cheaper to fly a replacement crew up to MAN on a regular service than to send them on their own aircraft.