I have a similar issue. It starts autodialing and auto playing music. I have a post on HTC's support forum and others are having whacked out problems, too
The first 3D I had would act crazy when I plugged in headphones. It would dial people, fast forward or skip songs. I exchanged phones. The present 3D activated the voice dial once, when I plugged in a head set, but I haven't had any problems since.
i dont think its your headphones. try connecting a 3.5 line in- line out cord to a car stereo, it should start autoplaying there as well. may have something to do with the audio player you have set as your default or most recently used. but ultimately im sure it is a hardware issue. im sure it was done on purpose