This can be caused by holding onto the poo for longer then you should. Cuasing the colon muscles to compress the fecal matter. Squeezing water and pushing it backwards back up towards the intestine.
Fecal matter after the initial steel head can be more runnier (softer) then usual.
Although it has happened for no reason at all. Its more commonly caused by holding on for to long or not enough water intake
"Just had this massive steel head.."
"Allan had a huge steel head this morning, poor bloke"