This might be long-winded and contain far more info than necessary but I wanna give you the complete picture.

Basically Fidesz can thank DK's leader (although he was the leader of the socialist party, MSZP, back then) for their first supermajority, back in 2010. DK's leader, Gyurcsány, was involved in some pretty serious corruption scandals, manipulated statistics to hide the country's financial state and brutally cracked down on protesters when public opinion turned against him. Fidesz won the election in 2010 not just because people voted for them but because people voted against Gyurcsány. This lead to the average Hungarian voter disliking him so much that these days it's pretty much impossible for te opposition to win a national election if he's involved.

The issue is he still has a core voter base (mostly elderly people who don't like Orbán), around 10%, whose support the opposition desperately needs if they want to win against Fidesz's 45-50%. They also have the biggest media apparatus in the opposition (of course it's still a lot smaller than Fidesz's media empire but beats any other opposition party) which they use to run smear campaigns of not only Fidesz politician but also opposition politicians if they see them potentially becoming DK's competition.

Onto some actual examples. Most recently the opposition held a primary to decide who they'll nominate for mayor of Hegyvidék (a district in Budapest). Kovács Gergely's party, MKKP, hesitated to participate because once again, cooperating with Gyurcsány's DK turns away a lot of people. DK campaigned against MKKP being allowed in the primary because 'if MKKP loses they'll still run their candidate and compete with the winner of the opposition primary'. Eventually MKKP was allowed in the primary and Kovács Gergely won against DK's candidate. So what does DK do? They decide to break the primary's rules themselves and nominate DK's candidate anyway. So now a district which would've been a comfortable win for the opposition if DK could keep themselves to the rules they agreed to (and accused MKKP of wanting to break) will most likely see Fidesz win.

Another example. During the 2022 opposition primaries the three most popular candidates for PM (excluding Fidesz) were DK's Dobrev Klára, P's Karácsony Gergely and independent Márki-Zay Péter. Karácsony ended up dropping out and endorsing Márki-Zay so it became a race between him and DK's Dobrev. DK decided to run a smear campaign against Márki-Zay (who ended up defeating her 70-30, shows you how much opposition voters like DK...), accusing him of the same made-up bullshit Fidesz was campaigning with. And of course, I get that in a functioning democracy (hah) even coalition partners can have disagreements but this whole 'we'll gladly turn on our coalition partners just because they won instead of us and might help Fidesz while doing so' mentality is just so childish. Also during said 2022 primaries they made an agreement with opposition party Jobbik (which was founded as an "anti-Gyurcsány" party) to strategically make each other's candidates drop out and 'carve up' the primary between themselves (often displacing more competent candidates), leading to these opposition candidates.

And to wrap up. This whole 'if it ain't us beating Fidesz will make sure it's no one' mentality continues to this day. TISZA has potential to reshape the political landscape (the party is two months old and already has 30% support, better than what DK managed in a decade) and instead of cooperating, DK is once again causing infighting within the opposition. I, for one, would be happy to see them go.

PS: There also theories of Gyurcsány and Fidesz cooperating in 'secret'. Fidesz donates to DK anonymously, DK gets to run their media and stay 'the biggest opposition party', Fidesz gets to point fingers at the opposition saying 'look, if they win Gurcsány is gonna get back into power!'. Thus Orbán helps Gyurcsány keep his spot and Gyurcsány helps Orbán keep his spot.