1) OK, let's go into the reasons why people are waiting longer and longer to be seated for their ADR
2) Sure, there are sporadic or random causes, such as
. . . extra hot days and people want more time to to cool-off
. . . there are several large parties and it simply takes a while for all to talk and eat
. . . folks sometimes just like to sit and kibbitz
3) 
Now, the real reason:
. . . people started to complain they could not get ADRs when they wanted them
. . . Disney does not like complaints, 
especially on social media
. . . at busy buffets, they altered the average table-eating-time (eg: "table turns" were reduced from let's say 40-minutes per table to 30-minutes per table)
. . . of course, people DID NOT speed up their eating habits
. . . so, those who might get the table next had to wait until the previous guests were done, the table cleared, and the table reset
. . . now, a snow-ball effect occurs, and waits get longer as the meal period(s) continue
. . . guests can see this especially in places like Crystal Palace, Chef Mickeys, Tusker House, or Ohana, where wait-times are sometimes horrific
. . . one seldom sees this occur at ala carte TS locations, as most restaurant managers will not allow the table-turns to be adjusted, and maybe have guests not return
NOTE-1: 
I can remember taking guests at my Concierge Desk who were charged the $10/person penalty when they got tired of waiting and walked off. 
Since the were not seated, they were declared as a No-Show. 
When I called the eateries, I was told of the Table-Turn adjustments. Now, manager that closes the restaurant has the final say-so if guests will b charged a penalty, 
However, we have some discretion, and I removed the penalties. 
Yes, I was "counseled" about such actions, because the hotel General Manager becomes responsible for the penalty and the effect on the P&L of the hotel. 
But, we still did it anyhow and hoped we did not get caught.
NOTE-2: 
The Lobby Concierge has more power at WDW than many people realize, or the Concierge will openly admit. 
I have given Wine/Cheese Baskets to guests, had balloons put in their room for a birthday or anniversary, had hard-to-get autographs written on photos and placed on their pillow. 
In some cases, I even traded SNACK-DDP coupons for TS-DDP coupons (and vice-versa) when folks did not have time to use them all. 
Since I had the Dining Override Code, I could fit people into eateries that were sold out or overbooked (except at true capacity-restricted events such as HDDR, LUAU, BBQ). 
In a few cases, I remember sliding parties of 12-or-more into a sold-out CRT, right in the middle of the meal period. 
We will not even begin to discuss how almost daily I moved people from rooms they did not like into rooms with a better view or more desired location.