Or just happen to stumble onto 4chan during the summer period of each year where it would be flooded with gore and other shock content*. Or be unlucky enough to be on a site which had been targeted by a raid by 4chan or other imageboard users and flood those with shock content.

* to provide historical context for the younger folk here, 4chan would receive a flood of new users every year roughly corresponding to the American summer holiday period, and there would be a noticeable decline in posting quality during that time.

To ensure that only people who fit in with board culture stuck around, the site (or at least /b/ which was where I spent most of my time back in those days) would be flooded with the most offensive content possible to scare away the bulk of the new users. This was rather effective at filtering out people of an emotionally sensitive nature and leaving only the most jaded and persistent to become regular users.

Much like the so-called "Eternal September", the board eventually had a culture change and became more welcoming of new users, and this practice declined. Although that was a long time ago and I think perhaps it may have been more that the introduction of CAPTCHA, rate-limiting, and other anti-spam measures prevented the use of automated image dumping software that was used to flood threads with shock content.