Frontline invites will come around if you fit the right qualifications and there are openings in the frontline pioneer program. As people let their status expire other players are invited.

Just curious. What qualifies you to be a Frontline Pioneer? Is it randomly selected by a computer, or do staff members hand select players once others leave?

It is an automated process, and I don't know exactly what the qualifications are. I believe there is a minimum rank of around level 40, and playing several games at one time for starters. There may be other factors such as trying several types of maps or playing with a variety of units.