Ecgfrith's two marriages—the first to the saintly virgin Æthelthryth (Saint Audrey), the second to Eormenburh—produced no children" (Wikipedia)
1) a general called Bert! Bert the warrior! Bert the slayer! All hail Bert!
(That little h really makes all the difference).
2) 'Other motives are possible'. I love that. Stag weekend that got out of hand...? I Know I Put My Keys Down Here Somewhere?
3) Only in one period does someone called Ecgfrith marry someone called Aethelthryth and we are expected to be able to untangle them. Maybe Bert's parents chose his name because they were reacting against the whole 'tongue in celtic knots*' problem. Also, wikipedia thinks Oswy had two sons, one called Aldfrith and one called Alhfrith. If this is true, it just seems like asking for trouble.
*perhaps more appropriately, Northumbrian Renaissance knots, but nobody ever says this.