I haven't read most of the rest of this thread, but, has anyone dealt with 'Bombing Along' for moving at a good speed?
The version of the story, as I understand it.
In the early stages of the Great War, the method of trench clearing involved riflemen and 'bayoneteers', each bay having to be cleared by fire power and by use of the bayonet (or less regular improvised weapons) when fighting corps a corps.
With the advent, first of large quantities of smaller grenades (as opposed to the longer and slightly unwealdy Grenade MkI<1>), and later of the Mills type grenade, clearing of fire bays and dugouts became a much quicker affair. The trench clearing parties now consisted of riflemen, bayonet men and bombers.
The use of grenades was known, in the British Army as 'bombing' (There had been some complaint - from certain Regiments with influence - about referring to the bombers as 'grenadiers').

Any other theories?

Walrus

<1> The Mk.I Grenade was originally issued for siege warfare (Royal Engineer issue only), it was in a long cane handle (unwealdy in the confines of the trenches) and were detonated by a percussion system, dangerous when throwing from confined spaces and requiring a good direct strike to detonate.