Bomber Stream
I am no expert - still researching - but based on reading, a bomber stream formed because a, they all had the same flight plans, landfall etc, and b, they all took off at an appropriate time. In Martin Middlebrook's book "The Battle of Hamburg" nine Lancaster's of 57 Squadron were allocated late take off and bombing times by mistake - they arrived over Hamburg 20m after the Main Force and took on the night fighters and flak on their own... every body else had been and gone. On the other hand the American bombers did attempt to form up before going on a daylight raid.