Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
TRIGGER
As described in historical dictionaries
Patronymica Britannica (1860)
TRIGGER. 1. I knew this name, at Alfriston, co. Sussex, corrupted from the local name Trigwell. 2. A correspondent informs me that this is a rather common name at Madeley, co. Salop, where it is understood to signify an employment. A Trigger is one who cuts small watercourses, locally called trigs, in meadows capable of irrigation. The name is therefore analogous to Ditcher.