In an unlikely chance that you let your wine ferment on wild yeast without killing it off with SO2 prior to fermentation, malolactic fermentation (MLF) can start on its own, without adding another yeast culture. In fact, I heard it can start on its own even in cases where you did kill off the wild yeast. The danger, however, of not following up with the addition of the dedicated malolactic bacterial culture (given that you want it to occur) is that wild MLF can pop up almost unpredictably during times you least expect it, may produce some off flavors, and can last for much longer time causing haziness and cloudiness in your wine. The worst is when it happens in a bottle.
So, if your MLF conditions are favorable (i.e. pH > 3.2, temp. > 64*F, and free SO2 is low), you can add the culture and go that path, OR, if you don't want the MLF to take place, inhibit it with lower temperatures and/or SO2 additions.