If you're using the rear, then the front is likely to release, particularly for that last squeeze at the lights, or when you have poor traction due to road surface (corrugations, or washboard are a classic example).
Because the wheels (particularly the rear) can't follow the surface of the road, and bounce up very briefly off the road, if you have the rear brake on, the rear locks, and as a consequence, the ABS kicks in. Sadly, the ABS kicks in at BOTH ends, instead of just the end which is locking up (i.e. if the rear wheel locks up, the front brake will release, and vice versa). BMW, in their infinite wisdom, did this to prevent riders from going over the top doing accidental stoppies under very hard braking.
The best solution to this is to fit better suspension components. Yours are probably at the end of their service life (that's techno-speak for "shagged") after 5 years and 34k miles. I have a bike of similar age and mileage, and my shocks are in a similar state. Better suspension components keep the wheels on the road better, and so get around this problem of the rear wheel losing contact, locking up, and the ABS kicking in. Alternatively, just use the front brake in situations like this, and that'll stop the rear from locking up as well.
