Clogged drain problem that doesn't make sense

[Markintosh]

Maybe its a volume issue. A toilet is only going to produce about a gallon of water per flush, which won't necessarily fill a 3 inch drain pipe if partial clog is down stream a bit. A sink faucet also does not produce all that much volume of water.

But a shower can produce a couple of gallons of water a minute, and maybe filling the drain pipe with water to the point where it is at the wax seal, causing the leak.

I would consider running a snake in the toilet.