If it's a military exercise then I'd be guessing it's chaff from a military plane trying to avoid a missile lock.
Doubt they'd be live-firing missiles though. Also chaff is usually dropped and then the plane turns the other way, not usually dropped continuously through 180 degrees (twice!)
Seems unlikely that it's condensation from a jet exhaust, given the abrupt start and stop nature of it, but stranger things have happened in the air due to weird inversion layers and the like.
So I've got nothing