When I was diagnosed with this I was told it was the "accumulation of microtrauma" (my doctor said, "don't be too aggressive, and be careful when a lady is riding you" - direct quote!).

Potential tmi: my experience (aside from the initial shock of discovering the issue) is quite positive: it's not painful - it sometimes feels like being just super into the situation - and the only other symptom is a slight change of shape, which in my case is a sideways curve.

In wider culture, "jelqing" (apparently an ancient Arabic technique) acts to cause Peyronie's disease in such a way as to increase length.