Common typo among people who like to insert many
exclamation points to the end of a sentence, caused by
accidentally releasing the Shift key before the 1 key, and sending the message without previewing it to realize the typo had been made.
Often used on purpose to
make fun of such accidents and the people who make them, or as part of leetspeak.
A similar but less-joked-about occurrence is the tendency to insert backslashes after a string of question marks.