Not paying the May 15 bond could have triggered so-called cross-default clauses, m eaning that holders of other bonds governed by foreign law that were also not swapped could have demanded immediate payment.
Their assumption seems to be that a real default (and along with it a triggering of the credit defaultclauses in all those CDS contracts) would be a bad thing for the banks and thus a credible threat to get them to line up for the voluntary version.
But there seem to be no clauses in the prospectuses for Greek government bonds that would trigger accelerated payment of the rest of what's owed on any particular bond in default or that would put other bonds into default: in the jargon, there are no accelerated payment or cross-default clauses.