One of our sites just dive-bombed from 8 to 191. But I don't see a reason for the penalty. I don't believe there is clear over-optimisation, the site doesn't have a huge backlink profile. Majestic SEO shows 38 external backlinks in historic index. There was a spike of 150 discovered backlinks in March, that is the only possible problem i see. However I have sites with much larger spikes that have no penalty and this spike was months ago.