Treasure is an attraction for all sorts of unsavory types. Dwarves, when they have lost their homes, leave behind treasure troves that thieves could only dream of having. The Skaven are an obvious treasure-seeking group, but it would be wrong to think that Clan Mors was the only Skaven clan to lay eyes on Dwarf territory. Another clan sought to claim the riches of the Dwarves- Clan Gritak.
Sadly, Clan Gritak ultimately lost out. During the war for Karak Eight Peaks, Gritak was betrayed by their brethren, enslaved by Clan Mors and eventually wiped out. It may be for the best- one Skaven clan is bad enough as it is!