Passive
* communication style
* a lack of self-reflection
* inadequate tests
* dishonesty (cognitive complexity)
Active
* prejudice
* dominance
* dishonesty (lying)
I often found that people are rarely introducing bias intentionally, though that does happen. The reasons people attempted to intentionally introduce bias was generally a way to improperly persuade people for a variety of unrelated motivations.
Bias can be as simple as confusion as to a goal, such as when a person states one thing but is actually hoping for something different. This is a disconnection between a person's goal and the terms with which they are willing or capable to express that goal. Reasons for that disconnect could be offense or cultural conventions. This is more easily observable in children who haven't yet mastered the communication skills to mask this confusion.
Sometimes bias creeps in because a person has a strong opinion of a subject and instead of attempting a proof to qualify or disqualify their position on the subject they will shift the evidence as necessary to manipulate an audience. This is common in politics, particularly when the audience themselves are preferentially biased to the emotionally loaded language they are hearing over any complete or partial evidence supporting that language. To some degree this sort of political manipulation is also most commonly unintentional, though it appears exceedingly intention coming from many career politicians.
Most frequently people introduce bias casually to achieve a form of social agreement, because facts and evidence are less important, to many people, than kindness and conformance. There is a sort security provided from achieving stronger social bonds even if that means shifting ground to conform to a different set of norms and values preferred by a group.
You wouldn't think any of this line of thinking would enter science or experimentation, especially when those conducting the experiments are experienced professionals, but it does. I have learned through other professional experiences that objectivity is a personality trait that can be tested for and measured. Objective personality types are rare. Everybody else needs defined boundaries and everybody needs some manner of peer review.