Based on his or her use of the term "trend" for both Pad Thai and Spicy Tuna, I'm going to assume Daemoness is asking about why these dishes (seem to) change universally.
As to the latter, I see no real way the chopped/minced version didn't at least start gaining momentum because of an economic efficiency. It seems to me (please correct me if I'm wrong since I am not actually a restauranteur) that sushi places spend as high or higher a % of the total cost of a dish on ingredients as any other type of restaurant that charges the same per customer. Whether a wholesaler or retailer came up with the idea is unknown to me (increased labor cost to make, without scale, is more inefficient for the retailer), but getting a cheaper costing tuna with little-to-no drop in consumer demand is the answer that makes the most sense.
The economic cause/effect for a saucier Pad Thai is less obvious. Given Daemoness's confession to infrequent ordering, it may be as much perception as reality. If it is a larger trend, then
excelsior wrote:3. Need to serve the most mainstream palate, which may be different to your preferences
makes the most sense. As far as the who/where of "Restaurant Zero" for the spread of the thicker, sticky, sweet sauce - I have no idea. Given that national chains tend to cater towards sweeter dishes based on the largest scale of market research (the Panda Express effect?), I wouldn't doubt it originated far from there.
Those with direct knowledge, of course, may prove my conjecture to be totally off-base though.