This sounds odd. Other than accidentally double clicking on a unit and giving them a move order (which would give a move order to everyone in that unit), I don't see how this could happen by themselves. I don't remember ever experiencing something like this. Did they come under fire when they would get into position?
Did that specific gun team spot the enemy infantry? The Brits may have been spotted by a different unit. Also, just because you're able to see something may not necessarily mean you can shoot it. Was the targeting arc you used purple or yellow?
If it's an HMG, then