Mine have done the same thing! Mine are in their second season and I believe that they were just multiplying so fast and there was not enough room for them to grow upright.

My Yellow Futurity cannas are doing the same thing. They grew super-fast and went waaaay beyond the normal height--(they are now 7 feet tall and still growing. They are all this tall, even tho I only planted them about 8 months ago). They are still flowering like crazy, and still healthy looking, only they are laying on their sides. LOL. I dunno if I should cut the stalk and just let a new one come up from the rhizome that will remain in the ground.