Basically, why immigrants who are only now being deported for crimes they committed 10 or even 15 years ago is because the change in the law in 1996 expanded the types of crimes that are considered deportable offenses.  The law was applied retroactively. This means that even though when you committed the crime it was not a deportable offense then--it is now--and you can still be deported.

More harshly, however, is that this law did away with the immigration judge's discretion whether on a case by case basis allow the immigrant to stay.  Click on the links below for more information on the immigration laws. 

A brief history of immigration legislation.

http://www.pbs.org/independentlens/sentencedhome/immigration.html

Current Immigration Law

http://www4.law.cornell.edu/uscode/uscode08/usc_sec_08_00001227----000-.html

Governor's Pardon Power 

The Governor's power to pardon comes from the state constitution (Art. III, sec. 9), which states: The pardoning power shall be vested in the governor under such regulations and restrictions as may be prescribed by law. The Clemency and Pardons Board was established to advise the Governor pursuant to RCW 9.94A.880 and 9.94A.885.