Active voice is generally stronger and more effective than passive voice, especially in legal writing. As William Strunk, Jr., advises in his always-excellent writing handbook, “[t]he habitual use of the active voice . . . makes for forcible writing. This is true . . . in writing of any kind.”
Many lawyers do not seem to know the difference between active and passive voice, and have a bad habit of being passive most of the time.
Active voice highlights the doer of an action: Sam sued a debt collector. Passive voice highlights the receiver of the action: the debt collector was sued by Sam. It is also easy to hide the doer entirely: the debt collector was sued. If that does not clear it up, Wikipedia has a comprehensive article on passive voice.
When you are advocating for your position, active voice is more effective: the debt collector violated the law. But passive voice is useful if you want to de-emphasize the doer or avoid placing blame directly (such as on the court): a recording of the debt collector’s threatening phone call was improperly excluded.
My general rule is to use active voice unless you consciously decide passive voice will be more effective. Go forth and be active!