Breakdown of Коллекционер наткнулся на редкость.
Questions & Answers about Коллекционер наткнулся на редкость.
Here наткнулся на means came across, ran into, or stumbled upon something by chance.
It often suggests an accidental discovery, not a planned search.
The verb can also have a literal meaning: to bump/stumble into something physically. But in this sentence it is clearly figurative: the collector unexpectedly found a rarity.
Because наткнуться на + accusative is the standard pattern.
So you learn it as a unit:
- наткнуться на что? = to come across / stumble upon something
- наткнуться на кого? = to run into someone
The preposition is not optional here. It is part of how the verb works in this meaning.