The Sunkensleep is a state that dwarves enter, sometimes willingly and sometimes otherwise, when they are too hurt to otherwise remain aboveground. They will disappear deep into the earth as if it is sand and sleep until they are healed. Depending on the nature of the injury, this can take anywhere from days to centuries.
Most famously, the entire population of dwarvenkind disappeared into the Sunkensleep 3000 years ago, and remained there for more than 2000 years. This was apparently due to a massively damaging tear in Verden itself, and the dwarves were unable to awaken until the world had healed itself enough for them to function.
Apparently, Verden is still highly damaged and it hurts them to feel it, so many of them remain in the Sunkensleep and those that are awake have a hard time with Dwarven Earthspeak.