It is possible that the pressure test was being conducted on an unused or new branch of the sewer, and the test plug leading to the active sewer failed.
Although, with the size and ferocity of the shit plume, it would have to have been a massive test. Here in the US, I use only 5 psi of air pressure, or ten feet of water column(which are pretty equivalent). For 5 psi of air to do this, the area of tested pipe would have to be very large, and the failure would have to have been instant.
