My guess is that she's rushing the divorce because either:
a) OM is putting pressure on her (unlikely); or
b) she is trying to maintain some sort of personal boundary by not sleeping with him again until she's divorced; maybe she's ridden with guilt about doing it once (or more) already?
c) her marriage to you really has been that awful (was it?)
Usually, it's either "a" or "b." Most do not rush to D this quickly -- most waffle and just throw the threat around a lot -- but off the top of my head I'd say maybe 20% do?
