I was cautioned by a station attendant when I was walking through the station premises while using my mobile phone, but I don't know why.

Operating a smartphone while walking through station premises may cause problems like colliding with other passengers or accidents resulting in a passenger falling onto the tracks. Railroad companies across Japan are promoting the safe use of mobile phones through joint marketing campaigns with cell phone companies. Mobile phones should be only used when you are standing still and in a place that does not obstruct the flow of foot traffic.