Fly Fishing Techniques

I am a relatively inexperienced fly fisherman having a casting problem. I took lessons last year, I have been fishing with a fly rod for about a year and I have logged hundreds of hours on the water to this point. This weekend I was traveling and had the opportunity to do some fishing along the way. For some reason, I was creating horrible twists which eventually got bad enough to create knots in my leader and tippet. I am not talking about getting an occasional wind knot or a little knot from bad casting, it was twisting badly enough that it was consuming 6 inches to a foot of leader/tippet and it was happening after 12-15 casts.

My question is: Does anyone know what I was doing wrong to cause this severe of a twist? I literally went through 3 leaders in a half hour span at one point. I have not run into this problem before.
The problem typically happened when I was fishing a Light Cahill Spinner in a fairly large size, they were taking sizes 12 and 14 very eagerly. I was using a 6X leader/tippet combo on a 5wt rod. I tried switching to a 5x then a 4x leader and tippet with the same results. I also tried different brands, so it wasn’t like I had a bad batch of leaders or tippet. I do not normally have the opportunity to throw that large of a fly and I was also casting pretty consistently to the distance limits of my abilities. Anyone have any suggestions of what my problem might have been?
UPDATE: Great answers and it led me to do some research this afternoon. (It was just another excuse to go fishing really.) I believe the first answer is correct. The big issue is indeed the fly. Here are my steps and results:

1) I started today with a big Elk Hair Caddis, I figured that the big wing would stabilize the fly somewhat when it was in the air, and began casting as far as I possibly could. Kept it up for about a half hour, no issues.

2) I switched to the same Light Cahill I was using over the weekend, within 5 minutes, I was knotted horribly again.

3) I then tied on the same pattern of Cahill in a smaller size, bought at the same shop and time as the big one. (presumably by the same tyer.) The small fly produced the knots, but not as badly.

4) I tied on the same pattern of Cahill, but one that had been bought at my home fly shop, I had no problems.

I did make things worse by speeding up my casts but I believe the bigger cause was the fly.