its usually because people on their phones aren't paying attention to whats going on around them e.g. tugs/lorries driving past. on more than one occasion i've had to physically stop someone walking across the ramp road whilst they were on their phone because they didn't see oncoming traffic. ground handling staff are told no-one should use their phone for this reason and due to 'health and safety' however no further explanation is given, thus why they couldn't give you an explanation