Lots of anti-virus programs will do that to anything they don't recognise.

I've normally got to disable AVG whenever I install a Dynomotion update, as it blocks installation due to the main executable not having been scanned by them, so they flag it as unsafe and block/delete it's installation. AVG do however give the option of uploading the file for further analysis, and it'll come back a couple hours later saying it's clear, so anybody trying to install after should not get any problems.