You're seeing interlacing artifacts. Interlacing is the process of capturing the even rows in one frame and the odd rows in the next. The alternative is progressive scanning which captures all rows every frame. Because only half the data is captured in each frame, interlacing is often used when a higher frame rate is desired, but the ability to transmit or store the data is limited. Most video editing software has filters to reduce interlacing artifacts, but you can't fix everything in post-production. If possible, shooting progressive scan eliminates this problem.