Aestivation is a normal activity of African bullfrogs in the wild. My African bullfrog buried itself regularly each year. It is a good rule of thumb to make sure your frog is healthy before it takes a nap. Unhealthy frogs may not make it. The frogs go down looking like a plum and come up looking like a dried prune. The "racing stripes" appear on juvenile frogs and will disappear with age.
You chose a really cool name for your frog, Keroppi, is that one of the Sanrio ("Hello Kitty") characters?
My female African bullfrog was named "Spot", really originalShe past away a few months ago after 12 years.