Do any youtube engineers have the scoop on why this happened ?
Probably because it removed "useless" search terms. I had to try it just to see if there was anything scary that slipped through YouTube. I highly doubt YouTube is intentionally harbouring pedophiles like these "alt-lefters" like to point out. Searching yields results such as "how to have the sex talk with your kids" which means the words "the", "talk", and "your" played no part in ranking what Google thinks you meant. Doesn't seem that far off to me.
Probably not a bug but just a common search phrase and therefore expected behaviour?