When Viacom bought Harmonix back in 2006 for $175 million, it was agreed that Harmonix shareholders would be paid bonuses if the developer exceeded performance targets in 2007 and 2008. The ex-shareholders claim that they were never paid the 2008 bonus, which would have been based on any profit over $42 million. As the Rock Band franchise's retail sales were over $1 billion in 2008 and the first quarter of 2009, the profits and payouts were probably substantial. The plaintiffs are still calculating a figure.
The group, which includes Harmonix co-founders Alex Rigopulos and Eran Egozy, further claims that Viacom worked against Harmonix to reduce the amount it would potentially owe. The suit alleges that during distribution agreement negotiations with EA in 2008, Viacom sought benefits directly for itself, rather than a reduction in fees that would increase Harmonix's profits--and therefore those of the shareholders too.
Furthermore, there's the issue of $13 million supposedly held in an account, which was due to be released to the shareholders but held by Viacom following a variety of rhythm game patent lawsuits. The shareholders claim they are still owed this.
Viacom will surely fire back soon enough and it'll all get awfully messy. The company revealed last month, before this news broke, that it was seeking to sell off Harmonix.