since the bike already had a broken spoke, I suspect the spokes were all stressed to their breaking point, or near to it. This is almost always caused by too little tension and the cyclical stress caused as the wheel rotates. If you get the wheel respoked and the tension checkedw ith a tensiometer (and probably re-tensioned after a couple hundred kms) then you will likely have no problems for years.

The reason the front spokes have not broken is because front wheels see much less load than rear wheels, and therefore need less tension to hold up.