Because that debris was an explosive device that looks something like a grenade in grainy footage and definitely blow’d up real good, hence everyone calling it a grenade*. This incident happened in Iran during the quarterfinals of the AFC Champions League between Sepahan and al-Ahli. You have to feel for the unassuming player who is just trying to clear the field of things thrown by his team’s own fans. A few seconds later, and he might have lost some fingers.
*This post was updated to remove the word “grenade” because Richard Whittall is right.