According to NBC 7 San Diego:

… water officials worried the damage might lead to the street collapsing.

Initially, the break was small but the situation escalated throughout the morning with the broken water main lifting the pavement in the area.

Water crews feared the break could collapse portions of the impacted streets, so traffic was diverted in the area as crews worked to shut off the water and begin repairs.

The cause of the main break is under investigation, but officials said the pipe is very old and made out of asbestos cement.