People are defensive when they feel threatened.
Most of the negative senses you refer to are about behaviour that shows a person starts acting as if they are being threatened when they are not. Such behaviour might be "being stubborn", "not answering questions clearly", "blaming other people", "making accusations against other people", "not engaging with the problem".
Consider a software developer. His boss wants to discuss options for making a certain part of his code run faster. But when boss asks him about this he starts saying "You can't change that code" and "I can't fix it because I've got other tasks now" or "It is the API designer's fault that the callbacks are inefficient", "it's your fault for not assigning me sufficient resources" etc etc etc.
Why is the developer behaving like this - perhaps he thinks he is being blamed and will lose his job, so instead of working productively he is being defensive and that isn't helping.
Some people get defensive very quickly, and it usually is a problem in interpersonal relations, since the defensive person is more concerned with "blame" and "who is at fault" rather than fixing a problem.
