Why are does ssn withholding go up so much every year?

self employed people (esp) have had a large tax increase that no one talks about. In 2000 after $76,200 there was no more SSN tax. In 2009 it's $106,800. That's 40% more income taxed. What is the justification for this? http://www.ssa.gov/mystatement/maxtax.htm