Here are some of my personal reasons.
The song reminds me of something very close to my heart. The reminder's cause... well may be the song used to play when that memory creation was taking place :) or may be the song is new but the lyrics have a message that hits the target.
We have a lot of playback songs for movies in India. There are a few songs which are brilliantly apt for the situation. If the situation is too emotional and has appealed to me a lot, then again this song is bound to cause goosebumps.
This one's completely opposite of the first. If I'm going through a rather emotional phase - happy, sad, excited, feeling low because of a failure, or extremely elated because of a success than I might remember a suitable song to hum :) and then goosebumps may be due to the song or due to the already emotional mindset.
I guess that's already a long list :D Gr8 Question.