Three people gored by bison in a month at Yellowstone National Park. Why do these attacks happen?

There have been three separate incidents of people being gored by bison in Yellowstone National Park in the last month. Park officials say people must keep a respectful distance or risk being charged by the massive mammals.

Three people gored by bison in a month at Yellowstone National Park. Why do these attacks happen?
There have been three separate incidents of people being gored by bison in Yellowstone National Park in the last month. Park officials say people must keep a respectful distance or risk being charged by the massive mammals.