Yep, do see this from time to time over the last month or so. And it happens at strange times. For example, I'll be looking at excursions on my cruise...then go to my order history to check the price I paid for an excursion (yeah, looking for a drop...), then go back to the main reservation page and click on Mange my Reservation on that cruise...and find it's locked. Then I just log out and come back later...
Interestingly, it doesn't happen all the time.
The only other time I've seen it locked was when I was working with a support rep...they had made a change and after I hung up, the reservation was locked...but this is normal...