Because it depresses the nervous system, and causes relaxation of muscles.
Usually after work, your body is tense and your brain's stress levels are higher. Beer reigning in this helps. A lot.
I'd suggest separating the batch and naturally carbing some while force carbing the rest. If the naturally carbed comes out non-metallic tasting, and the forced carb comes out metallic tasting you've found the culprit.
I haven't heard of anything that would make me suspicious of your process or...
Do you use a copper immersion chiller?
How long are you fermenting for?
Do you stick to the recommended dilution for the starsan and pbw?
What types of pots are you using (Aluminum or stainless)? Do you vigorously scrub them with steel wool?
Need some info on your process.