In Kate Bishop that he lost his hearing due to the constant explosions he's been exposed to as an Avenger. In the comics, however, it was a gruesome surprise attack that left him deaf.
In the 2012 Hawkeye comic series by Matt Fraction and David Aja, readers get a look at what life is like for Clint Barton when he's not an Avenger. In the series, he spends most days fending off the Tracksuit Mafia, a gang of red sweatsuit-clad gangsters with an affinity for calling everyone "bro." The gang's leader, who also happened to be the landlord of Clint's building, came after Barton when he bought out the apartment building to save the tenants from eviction, taking away the lucrative income source for the Tracksuits in the process. This puts Clint squarely in the Tracksuits' crosshairs, and ultimately leads to Hawkeye losing his hearing at the hands of Kazi Kazimierczak, also known as The Clown.
While the Tracksuits serve mostly as comic relief, the man they hired to take out Hawkeye is no joke. Kazi is a mercenary who has worked with New York crime bosses like Kingpin and The Owl. When Clint was able to evade his first attack, The Clown took it personally. After invading the building and killing Barton's neighbor, Grills, he launched a full assault on the apartment complex. While Clint and fellow Hawkeye Kate Bishop eventually take down Kazi and the Tracksuits, a violent assault by The Clown left Clint permanently deaf. In the scene, Kazi sneaks up on Clint and stabs him in the ears with arrows. It's incredibly graphic, and so violent it left Hawkeye in a bloody heap on the floor.
While a moment like that may have worked in the dark, gritty Clint's perspective as a man suddenly without sound. Both versions of his hearing loss origin have their place, and the one in Hawkeye certainly fits the MCU better, but it's a shame such a powerful scene couldn't be realized on the big screen.
Hawkeye's deafness has become one of his defining characteristics. Clint losing his hearing in Marvel Comics allowed Hawkeye's character to evolve and added depth to a hero often mocked for his normalcy, at least in comparison to his fellow Avengers. Even though that topic was still tackled in the MCU, the more PG approach stripped away much of the trauma Hawkeye had to work through after that violent moment.
