As they grow, babies may be distracted by their own movements. They may flail their arms or kick their legs, which can cause them to lose their grip on the nipple.
Tiny babies can only focus their eyes to approximately the distance between your breast and your face, so they aren’t distracted by other things they see around them.
The external stimuli could be noise, bright lights, or older siblings running around in the background. There’s a name for this phenomenon – the four month fussies.