UFO “fragments”? Meaning what? We shot one down, or one crashed, superior technology notwithstanding?
The “UFO community” explanation (fwiw—I am _not_ endorsing it) is that the 1940s and 1950s “flying saucers” could be disrupted by certain types of radar or atmospheric conditions, and there were crashes as a result.
If you want to read further on that topic the best book (currently rare) is probably:
https://www.amazon.com/UFO-Crash-Retrievals-Investigation-1978-1994/dp/0359685080/