One of the Mobs have run out of Squig Juice and will have no fuel for their vehicles until they can get back to their fort with the new supply they have just bought in town (Orks are so stupid that they will often run out of fuel before they realise that they are low!). The other Mob has just heard this news from a sneaky Grot informer and is seizing the opportunity to attack, attempting to bash their enemies and nick all their fuel.