It's a little bit acquiescence bias, but that's natural. Depending on the culture, participants usually seem to feel some pressure to be agreeable - no matter how many times you tell them there are "no wrong answers" or "this is not a test of you."
One method I use to try to take some of that pressure away is to break the ice and have a conversation with participants to hear about their background before the task testing gets under way. If you've done a little homework (I recommend it, since it helps people feel valued) you can ask them about their job or something you know they're very interested or involved in or experienced with. I find that people love sharing their expertise and knowledge.
And let interest be your guide, too. People pick up on scripted questions and they feel put-off by questions that sound rehearsed or read. If you ask them about things you're genuinely interested in, you don't have to worry about whether or not you come off as sincere.
The benefit in this context is that you're getting them into a more confident, natural head space and feeling more like themselves. I find this helps participants loosen up and not take the session so literally that they have performance anxiety.
(I'm sure there a word for this technique, priming or something.)