UIButton title changes to default

I feel like this is probably a stupid question... but anyway I have this kind of weird UIButton title behavior.

The button is set up and connected to both an action and a property in IB (the action is startButtonPushed and the property is startButton). Inside the view controller I have the action set up like this:

bool buttonStateStop;

- (IBAction)startPushed:(id)sender 
{
    if (buttonStateStop) 
    {
        [appD.locationManager stopSavingLocations];
        startButton.titleLabel.text = @"Start";
        buttonStateStop = NO;
    }
    else 
    {
        [appD.locationManager startSavingLocations];
        startButton.titleLabel.text = @"Stop";
        buttonStateStop = YES;        
    }
}

Originally I had the default title in IB set to "Start" but whenever I pressed the button it would change to "Stop" for a fraction of a second and then back. I spent a while trying to figure out why the title kept getting set back to "Start". Eventually I changed the IB title to "xxxxxx" and realized that no matter what, the IB title gets reasserted immediately after the title of the button changes.

So the question is: why does IB keep changing the button's title back to default? I've never come across this behavior before. And (obviously) how can I fix it?

Extra info: the only references to the button are the @property, @synthesize, and the statements in the code above. The view is inside of a navigation controller.