I don’t have comprehensive data to back it up, or anything like that. But the last time I looked at these figures for individual colleges, 6-7 years ago, the yield rate was 98%+. Many colleges enrolled 100% of ED acceptees, and the others had a difference of 1-5 people between acceptance and enrollment. I think the current numbers reflect a higher willingness to back out of ED.
That may be because of increased expense and less adequate financial aid. But it may also be because the colleges are being more successful at getting applicants who need aid to apply ED. The more price-sensitive applicants you have, the more you may accept, and the more you accept, the more who are going to think their aid offer (or lack thereof) is a deal breaker. To the extent that’s the case, the slight increase in the number of applicants turning down an ED acceptance may be a sign that the system as a whole is healthy.