Why treated as a spam??

Got an email from Z, telling me that my email was classed as a spam and went straight to his junk folder. Why? I have no idea. He reckons that it happened because I typed "video" among the words in the subject line. Did I write anything dirty? No, I don't think so... Well, I write loads of rubbish, but they don't need to class my email as junk, you know...