Black Cats supporters took to a Sunderland fan forum after the match to reveal details of the incident, which is said to have taken place at the Stadium of Light during the defeat.
According to Daily Mirror,writing online, one fan explained how the man sat in the South Stand suddenly jumped to his feet and dropped his trousers.As fans sat near the man noticed what was happening, they began to move away from the area.
The supporter is then said to have pulled his trousers up as stewards escorted him out of the ground.
One fan, reciting what happened, added: "A bairn (slang for a child) in the row in front saw the s*** and started gag.
The incident generated a strong response from many other Sunderland fans, who expressed their shock on social media.
