The most likely way to burn transmission fluid is through the crank seal, it's a moving part and the rubber can go bad and simply fail. Damage throughout the years on either the case sealing surface or crank sealing surface is also likely causing a shortened life in the seal itself.
There is also the possibility that the case is leaking between the halves. Some portion of the crankcase is exposed to transmission fluid and the only thing holding it back is the case sealant. It's not very likely that it will start leaking out of nowhere but much more likely on cases which were just put together and the sealant either didn't take the case halves (because of contamination or poor sealant technique).
The best way to diagnose the problem is to remove the clutch case and primary drive gear and do a leak down check. Once you've confirmed you are losing pressure, you can spray the seal lip with soapy water. If it bubbles out of the seal itself, obviously you need to replace the seal. If it doesn't bubble out of the seal but you're still losing pressure, it's possible the cases are leaking.
