Probably because somebody else was crossing it:
We investigated social information use by pedestrians in a potentially dangerous scenario: at a road crossing. To judge a safe gap in traffic, pedestrians can use social information, such as the crossing behavior of others, and follow others across the road. We tested if pedestrians followed others in this scenario by analyzing pedestrian starting position and crossing order. First, we found that neighbors of a crossing pedestrian tended to cross before other waiting pedestrians and that this tendency was significantly higher in observed pedestrians than in a null model: a simulation in which pedestrians did not follow each other. Also, by fitting the null model, we found that on average a person was 1.5–2.5 times more likely to cross if their neighbor had started to cross.
Link to study.