I'm fairly sure YouTube is very vigilant with enforcing adherence to copyrights. In Germany any video using music that is under GEMA license (the media licensing authority in Germany) will be blocked. They don't just silence the video, they block it altogether. I think this is done in other countries too. It doesn't surprise me that you're having this problem.

As to why yours was muted and others weren't, it may just be bad luck or you used music that was easily recognized as being copyrighted.

You could try alternative video hosters. I think Dailymotion is also very strict, but I've found that Vimeo is so far rather lenient. Their big caviat is that if you have a free account, it can take up to an hour for them to process your video after it's uploaded.