As a director in a public company, he is very restricted in when and how he sells his stock. In one case I know of that meant placing sell orders 3 - 6 months in advance regardless of the stock performance.
"These plans are widely used by officers and directors of public companies to sell stock according to the parameters of the affirmative defense to illegal insider trading available under Rule 10b5-1, which was adopted by the SEC in 2000."
http://corpgov.law.harvard.edu/2013/02/05/rule-10b5-1-plans-...
A perfectly legit 10b5-1 which would have resulted in exactly what we saw here. In February Barron's reported Andreesen's sales as being directed by a 10b5-1 plan: http://www.barrons.com/articles/SB51367578116875004693704580... so he's clearly familar with the process.