I suggest you ask your teacher to explain what she means by choppy. But it may be because you contravened the given-new principle, whereby consecutive sentences begin with known or already mentioned information and are followed by new information.
Consider the following simple example:
The second world war began in September 1939. The invasion of Poland by German troops caused it. The Polish Socialist Party governed Poland at this time.
The second and third sentences each start with new information, which results in a choppy effect. Using the passive facilitates the given-new pattern and makes for smoother transitions from sentence to sentence:
The second world war began in September 1939. It was caused by the
invasion of Poland by German troops. At this time Poland was
governed by the Polish Socialist Party.
There is a very good article on the passive by the Writing Center at the University of North Carolina: Passive Voice.