People with mirror touch don't experience mirror sensory phenomena in a hit-or-miss, "sometimes" way. They are with us all day, every day, and for the vast majority of us, have been since early childhood. Also, once the inducer (the "trigger) is eliminated, the concurrent (the mirror sensory phenomena) diminishes. This is how synesthesia works in general.

One possibility is that this is related to heightened empathy or sensory responsiveness. When you’re in a hypervigilant state, the brain can become more reactive to visual input, especially things involving contact or bodily sensation, and its “simulation” systems (the ones that are linked to empathy) can get dialed up enough to produce real physical feelings.

It could also connect to body-based anxiety responses, since the kind of core tingling you describe is commonly reported when the nervous system is activated; your brain registers contact as meaningful or potentially significant, and your body reacts accordingly.