In Prussia at least, the Junkers (I mean the general nobility, not just the East-Elbian settlers) basically did control the government for the majority of the time. An example of that being the three class voting franchise. For there to be a Junker coup, I'm guessing they'd have to lose their control over the government first.
The three-class-voting only worked for the nobility in the places, where their top-spot in society was uncontested, i.e. in VERY rural and backward places which are generally now part of Poland. Everywhere else, the bourgeoisie profited from it because they payed a whole lot more taxes than an old conservative Von Soundso who pushes a few peasants across his fields.
To make things clear: there is an anecdote that in a certain well-off district of Berlin where a lot of politicians lived, they actually all had to vote in the 3rd class because not only the first, but also the second class fell to an extremely rich sausage-millionaire...
Then, the Junker would never be in a position where it is theirs to rebel against the King (of Prussia who from '71 onwards happens to be the same guy as the German Emperor). They could try to exert influence in certain questions when a king's position might become untenable and another option a possibility to them. But generally, they were OK with what the Hohenzollern provided. They could support a putch to preserve the monarchy, e.g. if Socialism looms around the corner. But - you couldn't call that a Junker's coup because so many more interest groups would be involved. Again, the bourgeouisie, the armed forces (which could for a long time be associated with the Junker-class, but less and less so with time) etc.
In the vein of
supporting the King one could say that a Junkers' coup happened in 1862 when Bismarck was presented to a King who was at the verge of abdicating due to Parliament not financing an army-reform as "a guy who could solve things". Bismarck sidestepped the Constitution with verve and prevented a more liberal Prussia. D'oh!