I suppose in a very simplistic sense, its a case of AA coming into the party after years of UA having the monopoly on the market. Consequently, if they want in, they will have to commit!
Whilst UA has a 92% LF and AA has a 66% LF, in simplistic terms, what that does tell us id that 15,340 pax travelled between BHX-NYC, a massive increase on last year. Also if the load balanced out, that would give each airline a LF of just short of 80% which again is pretty healthy.
In very simplistic terms, this shows that there is a market to sustain both routes, but if AA wants to increase their market share, they have to either a) tempt loyal UA pax over to AA or b) increase the number of pax coming to BHX in the first place... I hope the latter wins out!!