^
Arrogance/hubris is a nasty thing. World Cup history is peppered with spectacular downfalls resulting from it: Brazil in 1950, Hungary in 1954 (more like over confidence), Netherlands 1974 and arguably France 2006. Netherlands in 1978 wasn't far off either, the Argies made a good job of getting under their skins.
As for Hungary... they had been pissing on every side in sight for years. They had just won the 'Match of the Century' as you say against the only side unbeaten in World Cups and which they expected to be the biggest test for them. They had won 8-3 during the group stage...
Over-confidence, tiredness after a grueling ET win, and thus relaxing and taking their eye off the ball to some extent.
There's huge merit in Germany coming back for more after that 8-3 and succeeding, but don't underestimate how much the Magyars switched off.