"To trigger an investigation" means to cause or initiate a situation where an official examination or inquiry is conducted to gather information and determine the truth about a particular event or occurrence. Full definition
In most criminal harassment cases, the complainant contacts the police with information about the actions of the accused which triggers an investigation that may lead to charges being laid. (vancouvercriminallaw.com)
There is usually little follow - up unless something like a cheating complaint triggers an investigation. (huffingtonpost.com)
Downing Street should relinquish its power to trigger investigations into ministerial misbehaviour, MPs are demanding. (politics.co.uk)