Just means there was a lot of CO2 suspended in solution. This would have happened right after the vial was packaged since it is usually fermenting some sugars in the wort it is stored in. Whether the yeast was dead or alive when you opened it, the CO2 will still have a tendancy to want to leave solution, like if you open a pop bottle. Difference is that there are proteins in the wort that will cause extreme foaming.
Now, given that most people will tend to shake the vial to keep all the yeast in suspension prior to pitching, this just exasperates the problem. Think of shaking a can of beer before cracking it open.
