There is a parallel to this Celtic imitations that is found primarily in modern Ukraine[1], attribute to Cherniakhov culture[2]. The theory for them is that once the trade with Roman Empire ceased, the locals needed bigger supply of coins and started minting their own.

There is a curious thing with this "branch", I'm not sure if it's the same in the Celtic one. The last time I talked to people researching this, I was told that: a. The findings are mostly unique, it's hard to find two copies of the same coin. Sometimes obverse of one coin could be found on another, but reverses don't match. b. These coins are not cast, they are minted through "hammering", which requires a stamp. However, not a single stamp has been found so far. A much easier way to make currency out of existing one would be to just slap existing coin into some clay, make a casting mold and just pour molten metal into it.

This of course is more of a curiosity/rumor level, I don't have any qualifications to back it up.

[1]: http://barbarous-imitations.narod.ru/ (apologize for a .ru website, but it's the best catalogue to my knowledge.)

[2]: https://en.wikipedia.org/wiki/Chernyakhov_culture