Sewage line coupling leak
I have a question about a leak in a sewage line. A cast iron pipe collects waste from a single toilet and runs down 5-6 ft vertically to the basement where it fuses with the house main sewage pipe through an L-shaped cast iron segment of the same diameter (2 ft vertical, 90 degree turn, then 2 ft horizontal). The rubber coupling connecting the vertical part of the L-shaped segment to the toilet pipe suddenly started gushing water after each toilet flush. Nothing else causes this leak, just that toilet; that is, the rest of the sewage runs okay. The toilet flushes normally, no standing water. I suspect that the horizontal part of the L-shaped pipe got dead clogged. Do I have to remove the toilet to clear the clog with an auger or applying some sort of chemicals would be possible? Why would this 12 in. pipe get suddenly clogged? Thanks in advance!