Google ranking, sudden drop

Could it be https:// redirect?

[wheel]

I've just started doing SEO for one of my clients (who's agreed to let me treat him as a test case). Things have been going well so far, but today his results plummetted.

It doesn't look like an algo change, as everyone else stayed pretty much the same. The only things I've done are
- develop more inbound links (all relevant one way afaik)
- removed an automatic redirect from http:// to [....] Previously if you typed in www.widget.com you ended up at [widget.com...] (the secure/encrypted page). All links were to the http:// site.
- I added a one way outbound link to a competitor, but think that may have been after the drop.

Is it possible that removing the redirect has caused this problem? If not, how does one go about testing what I broke :)?

TIA