Bokito may have become frustrated because the woman, who appeared to show interest, repeatedly walked away. Van Hooff speculated on what might have happened had she remained still rather than resisting. “I think he would have given her a few slaps on the back. And then, yes, he might have done what male gorillas do with female gorillas.”
Bokito was the reason for these, and the dumb woman he attacked.
If I recall rightly, she went to the zoo repeatedly to lock eyes with the gorilla because she believed that they were in love.
From Wikipedia:
Bokito may have become frustrated because the woman, who appeared to show interest, repeatedly walked away. Van Hooff speculated on what might have happened had she remained still rather than resisting. “I think he would have given her a few slaps on the back. And then, yes, he might have done what male gorillas do with female gorillas.”