How do patients get burn marks on their back when they get lipo? Is the cannula hot?

Having used most modalities of liposuction I think the advantage of a laser or ultrasound is not there but potential problems, such as burns to the skin, do exist.

It is rather unusual to get a burn mark on one’s skin after
liposuction. If you are having an
additional procedure and an electrocautery device is used, this can be the
source of a burn mark if a hot Bovie is left on the patient’s skin. Alternately, there are several new
liposuction technologies, which can leave a burn mark on the skin. These burns are typically created from the
inside-out. Best of luck.

Yes, with laser assisted  liposuction (LAL) and ultrasound assisted liposuction (UAL) that is exactly what can happen. Surgeons must carefully monitor the skin temperature as a result in order to prevent burns. Some systems, such as the BodyTite have internal temperature monitors that turn off the energy before burns can happen, making for a much safer experience. Ask your prospective surgeon what system they use and how they prevent burns. Hope that helps!