didn't the yahoo stock climb to about what MS was offering once the news came out? The disgruntled shareholders could have sold their shares then and gotten their 62% premium.
I would also think that an implicit part of their thinking is that if YHOO does not take MSFT's offer, the market bump in the share price that was caused by the offer will go away and the shares will be trading at sub $20 again.
Stephen Bainbridge has a good assessment of this and similar suits with additional info at: http://www.businessassociationsblog.com/lawandbusiness/comme...