I believe it's the shock, awe, and arousal of surprisingly finding out that a woman who may look average in appearance is actually your preferred type, physique-wise, when she exposes serious muscle development that's concealed by her clothing. I've experienced it one two many times from former female co-workers. Oh, how those memories still linger. Living in regions where you actual experience all four seasons is good for that type of experience.
Guy with camera sounds more than impressed