I think TJ’s theory is supported by reading I’ve done on this topic where any (and there is little) research has been done – Predominantly in automotive circles, but I can’t see any fundamental differences between the processes in auto brakes and mtb brakes (other than disc material, which is really a kind of substrate for deposition of pad material in a ‘correctly working’ brake.
The idea is to cure the pads initially under pressure and heat to allow them to deposit a micons thick layer of pad material on the disc and allow the pads to mate fully with the disc surface. This facilitates adhesive friction between the two layers of pad material
After that, during continued use the pads need to get adequately hot to renew/topup the layer of pad material on the disc. Otherwise the layer of material on the disc gets removed and you’re relying on abraisive friction – generally a weaker friction force and much more erosive on materials (both disc and pads).
This is why bedded in pads can feel great to start with and get worse with use if they never get hot enough again.
Sound familiar? Rear disc brakes in the dead of winter any one? Pads disappearing in one ride because they’re not getting hot enough to get past the abraisive wear/friction stage?
The inference is that disc brakes need to be run hot hot enough to continue the pad material transfer process, but not so hot that they boil and fade.
This would suggest that many of us are running discs too large that don’t get hot enough, often enough after our initial efforts at bedding pads in.
Glazed pads (if they are actually glazed) are a function of this, but are effectivly being polished shiny due to the inadequate force and heat being put into them.
They’re either not getting hot enough for long enough, or are contaminated.