How Can I Help My Driver?

Solution 1:

You get the notification when your driver is under fire from another turret.

Be sure to position yourself so that you can shoot players in other turrets and stop them from hurting your driver. You will be able to see other players controlling turrets marked with their screen name:

In the picture above you can see that AridSpider and coma buNNy are in turrets that are attacking drivers (friendly marked in green). Firing at these turrets will prevent other players from using them, saving your poor driver.

This video has many examples of the notification popping up, all when the driver is damaged by another player's turret.

Solution 2:

I was under the impression it happens when your driver is under fire, but I haven't confirmed it.

Most of the time, I 'ignored' it by just trying to blast away competition (turrets first then cars).