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Could be a bunch of things

1) I think you have a bad card driver. Either the radio that is sending the CTS frames has a corrupted driver or maybe the card that is actually doing the capturing is misreading the frames because it has a bad driver. I have seen this happen before and you might want to try and capture the traffic again with a different card and maybe even a different sniffer.

2) Any chance you updated any firmware on a client card, controller or AP? I have also seen new vendor code updates completely mess up 802.11 captures. The vendor does something goofy with the frame and all of the sudden the protocol analyzer cannot interpret the data properly. This is rare, but I have seen it happen.

3) Also, absolutely look at those CTS frames and see if the CRC failed. It is entirely possible that the frame transmissions are being corrupted and the protocol analyzer is simply misinterpreting the information due to the corrupted frames.

4) It could to a Denial of Service attack from someone, but I think the three above scenarios are more likely.

I am sure there will be others that will chime in about the CTS frames you are seeing.