Likely a calque of Dutch met zijn gat in de boter vallen.

land with one's bum in the butter (third-person singular simple present lands with one's bum in the butter, present participle landing with one's bum in the butter, simple past and past participle landed with one's bum in the butter)

1. (chiefly South Africa, also British and Australia, informal) To find oneself in fortunate circumstances.
   • 2011, Bill Green, Billy: One Boy's War, page 56:

     Margaret and Eric, the brother and sister who moved out of our house really landed with their bum in the butter. They were fostered out to a family that had a car which meant they were rich, as only rich people had cars.

   • 2012, Roger Lucey, Back in from the Anger, page 151:

     Once again, we'd landed with our bums in the butter. We spent the next couple of months in New York at shows, listening to music and meeting a dazzling array of artists, musicians, playwrights, actors and others […]

Most common in South Africa, although according to the Oxford English Dictionary it has also come into British and Australian usage.

• avoir le cul dans le beurre (French)