Why did plugin activation change the PHP on all plugins?

I have been building a site with a number of plug-ins just fine, but I just downloaded “YouTube Channel Gallery” (seemed to have good reviews), and suddenly I got an error message listed for each plugin, something like having an “invalid header”. And then, when I clicked again, all of them just disappeared!

I went and checked the plugin folders, and all of the php files within each plugin have had their code modified on the top line! I am going through and replacing all of the folders with a recent backup, so crisis averted… but I am nervous about installing more plugins. How in the world would it alter the code on all of my other plugins? Is this a danger for that specific plugin, or was it some sort of error when activating it?