Fibrinous inflammation, exudate and scarring in monotonous movements and ageing
I would now say that the hardening is just local acute venous congestion if in the short term.
Without stretching after practice, it just takes more days to recover.
So lymphatic drainage is the one which helps the muscle get its needs and nutrients.

If this happens during years, it is difficult to say what is the causal factor.
It depends on many factors. 
One key factor is how well the lymphatic circulation can recover the body back.
So genomics of the patient comes in the picture.
Many other factors too like anatomical structure of chest.
Then, depending on these factors and other pathological processes may happen many processes like


accumulation of fibrinoid
accumulation of calcium deposits
shorter diffuse distances
smaller total area of cells


which are some adaptation processes to the ageing (also of prolonged monotonous weight lifting).
They can lead to


acute lymphangitis (reddening of the skin)
Chronic lymphangitis
Chronic Granulomatous Lymphadenitis (fibrosis happening between muscles and connection points - not in all cases granulomatous - rare only)
lymphadenopathy (very, very late - after many years)


during ageing (adaptation).

Better question would probably be: what are processes happening during ageing?
And then cover pathologies of ageing separately and also the physiological processes on the weight lifting separately.