it's so weird lol
it's probably like when you see or hear someone yawn you have to yawn to or possibly even talking about yawning because i just yawned lol but instead of yawning for you its when someone cracks a part of their body ha
Same reason you feel like yawning whenever someone else does - mirror neurons. They basically make you want to imitate a behaviour.
it's a subconcious thing, basically you mirror what the other is doing
Its probably a habit you've probably developed.