Cracks in soil stack

I was trimming a couple inches from the soil stacks on my roof to fit some lead flashing and noticed there were multiple longitudinal cracks around the circumference at the end of the pipe. These are cast iron vents, and the cracks were there before cutting (the fracture surfaces were corroded).

My question - what stresses caused these cracks? It’s more of a curiosity since there’s nothing I can do about it, and the pipes are still relatively intact for the 46-year-old house.


They were probably there since new.