This occurs usually from four activities:
(1) Contact tonometry (both applanation and indentation - the instruments must touch the cornea to get IOP readings)
(2) first-time contact lens trial
(3) slit-lamp biomicroscopy
(4) Insertion of punctal plugs
In all situations, once the patient's eyes or eyelids are touched by the tip of the tonometer, by the doctor or an assistant when inserting a contact lens, or by the doctor's pulling down the patient's lower eyelid for a closer view of the palpebral conjunctiva or the puncta, usually at the slit-lamp, vasovagal reflex kicks in within seconds.
Interestingly, most patients are young and healthy, possibly for the first time in their lives, their eyes are now manipulated by a stranger. The eyes/eyelids are indeed under some mechanical pressure albeit quite minor yet the physical reaction is quite dramatic.