Sometimes as roses mature, they send up relatively tall canes. It could be the Knockouts are getting taller as part of their natural growth habit in your area.

When I see these tall canes in late summer, I wait until spring to prune them back.

Thanks for the info. I pruned the tall canes way back about 4 weeks ago (along with the rest of the plants), and they have shot back again. Is that normal if they are just part of the natural growth habit? (I did fertilize right after pruning.)

I have heard reports of Knockouts growing large in a warm zone. So this could be normal. It sounds like they are producing healthy, vigorous growth, so the fertilizer is not likely a problem.

Sometimes roses surprise us with their growth habit, they can grow larger or smaller than what we expect.

I took pictures of a knock out rose a couple of years ago that almost acted like it wanted to be a climber; it put out long shoots. Does yours look like this?

{{gwi:203768}}

No. It's different. The shoots go straight up and the stalk is very thick compared to the other branches on the bush.