I don't think that Hungary is being singled out specifically. As someone said, the change fee on that fare exists out of (at least) the Czech Republic too. What route do you typically fly that you are seeing this fare on? I'd be curious to look it up.
Just for fun, I looked up a R/T fare from BUD to LAX on 1/29 returning on 2/2. The cheapest business class fare is INCEU7, which carries the 300 Euro change fee and prices out to a total of $3700, which is a pretty good price (appears to also have a 7 day advance purchase). For the same dates, the unrestricted business class fare is JR, which obviously has no change fee and prices out to a total of $8400.
So I don't think it's a matter of "the D fare all of a sudden has a change fee." It's more a mater of price versus restrictions. If the fare you are seeing is $8,000 and has the 300 Euro change fee, then I'd say that's pretty unreasonable. But if it's only around $4,000 R/T, then there's a good reason for the change fee, regardless of what inventory it books into. So again, it's more a matter of how much the fare is versus what the fare basis is.
I'd appreciate if you could tell us what dates and routes you're look at so that I could look this up.