Here it goes... when you are dieting down your cortisol is already a bit higher (it is needed to mobilize stored energy), So if you do too much volume, especially "demanding" volume you risk jacking it even more.
So now you have chronically elevated cortisol. In the short term it can actually help you lose fat (it increases fat mobilization) but when it is chronically elevated it will slow down fat loss. How?
Cortisol can reduce the conversion of T4 (inactive thyroid hormone) to T3 (active thyroid hormone). So over time, chronically elevated cortisol will slow down fat loss. On top of that I increases the antidiuretic hormone, which makes you retain water (making you look fatter). So then the natural reaction is to do more and eat less... the result is only jacking up cortisol levels even more.
So yeah, getting lean is not simply a matter of doing more in the gym.