The 1950-D nickel had nothing going for it beyond low mintage. People hoarded them aggressively.
The Mint had two problems in 1965. They wanted silver and clad to circulate side-by-side. It was probably already becoming apparent that many citizens were pulling every silver coin they could find due to climbing silver prices. If collectors started pulling all the 1965 clad coins as well, because they were a first year coin, that could cause a serious coinage shortage for dimes and quarters.
I think it's important to keep in mind that these situations could resemble bank runs and tulip manias. It doesn't take a huge reduction in the supply to spark a craze. The first day that people and businesses couldn't get coins at the bank counter to buy train tickets, groceries, and smokes from vending machines, that could trigger enough widespread hoarding to cause a real economic problem. Insufficient circulating currency is a bizarre phenomenon, and the Mint probably just wanted to reduce the hazard any way possible.
Also don't underestimate political pressure. There was some bad blood over the 1950-D fiasco. A Mint director who was asked about the dangers by Congress, and then didn't take every step to reduce the perceived danger (no matter how distant or silly), would lose their cushy job in a heartbeat.