Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of MGIC Investment (NYSE: MTG) surprised us all today by climbing as high as 10.1% on about twice their average daily trading volume.

So what: MGIC and fellow mortgage insurer PMI Group (NYSE: PMI) posted strong gains, while Radian (NYSE: RDN) got its jump yesterday and stayed back today. None of these companies has reported earnings this week, and the housing data released today was only more cause for gloom in the mortgage sector. There's absolutely no reason for these stocks to soar here, other than perhaps a bit of enthusiastic bottom-feeding and/or market manipulation.

Now what: It's comforting to see the stock take a break from exploring new 52-week lows, as MGIC has been doing since January. Then again, MGIC made a similar leap at the start of July based on a slightly favorable credit rating on PMI Group -- tangential news at best, and nothing to get too excited about -- only to lose it all and then some two weeks later. If you're actually investing in this sector, I hope you know something about mortgage insurance that I don't, because the industry still looks like a surefire place to simply destroy your investment dollars.

Interested in more info on MGIC Investment? Add it to your watchlist, you market masochist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.