I do not know, but I suspect she was mentally ill, but nobody noticed. This certainly can happen.
The officer did not recognize this or did not know what to do about it. I could not see or hear much of what Bland did while in the car. But when she got out, she said a lot that did not make sense. The officer was angry. But she was taken into the jail apparently without injury. So the officer may or may not have behaved unprofessionally, but she was not harmed there.
He did say that she did not cooperate with the warning he attempted to give her; and this seems correct.
In the jail she is reported to have said she was depressed, and she still seemed to be confused about why she was there. Yet she said she was fine, and they believed her. Until they found her dead.
As I say, I suspect she was suffering a mental breakdown that nobody recognized. If she had been watched more carefully, she might be alive now. But it is hard to put the blame on any specific person or persons, since even her family and friends did not think there was anything wrong with her.
I could be mistaken, but this is how it looks to me.