DCT holds gears longer in sport, mode 1 upshifts at 2500 or 3000 while "loafing" if I remember correctly. You say you were also going downhill which tells CPU to hold gear even longer. Maybe the the bike shifted, then you started the decline so it shifted back and it sounded like three shifts? DCT can't predict your actions nor see the road ahead, for this reason I often ride in manual in all but flat smooth traffic.
1-2 is definitely the loudest shift, (maybe from N is more), since it still is a manual gearbox after all.

 

I may have been in D-mode actually. I always use S-1 when I remember, which is as soon as I hit the road because it shifts so quick in D. I was doing parking lot speed, slow, barely twisting the throttle. That could be an explanation though. Maybe it upshifted then instantly downshifted, but that would only be two shift "clunks". Maybe I was in 1st when it started, I was going real slow not looking at display during this.

But I had never before that had multiple quick sounding shifts, or since...

I like manual mode alot too. I always forget to downshift, not a problem when you come to a complete stop. In the slow corners it makes for lugging and bucking for me.

 

I have had this happen to me today - was in second and 2 or 3 gear change clunks but did not change - after that there was a few more seconds and it finally got into 3rd. (This was in Sports 2)

I tend to leave the bike in Sports 2 and flick down gears with the thumb for approaching hazards, corners, etc. as like yourself I tend to forget to shift down when in manual mode always leaving it hard to do anything when actually commited to the manouver.