I don't think one can have fear without any apparent reason.

Your nervous system will alert you (sympathetic nervous system will trigger stress response, amygdala signalling danger). However, there is also the interpretation you/we put onto what is happening to us. This cognitive appraisal will turn a purely physical sensation into something more. A physical reaction would die down if there were no 'reasons' behind it.

IF one has an extreme or disproportionate reaction to what happens (stimulus triggers our alarm system), I believe there must be an underlying reason... one we may not be conscious of. Reasons for that may be that something has been repressed (e.g. too traumatic to recall in full awareness) or something that happened to us at a pre-verbal stage or when our cognitive development wasn't mature, so to speak.

All that to say that I think what you saw triggered a body memory and didn't just randomly happen. There was a meaning underlying your reaction, one you may not have conscious access to yet.