As others mentioned, trigger means something that sets off a medical condition. It's still widely used without this fuss for many conditions, like my DD saying the recent poor weather has triggered the eczema on her face.
We have an odd mix of a society that has become over-pathologised so medical terms get overused and warped in common meaning, a 'be kind' force that uses that triggered is connected to disabilities to enforce them well beyond their scope to simply anything that causes an unpleasant emotion, and the bootstrap brigade which mocks displays of unpleasant emotions or disagreeing with them as 'being triggered'.
I'm not sure significantly more people are triggered in the mental health sense, it's just being used a lot more in many other ways.
Triggered warnings are helpful
No they aren't.
Content warnings like we've had on movies and video games for ages can be useful for anyone to make an informed choice.
Trigger warnings often unhelpfully vague (it's not uncommon to see 'trigger warning or TW with no additional information), cannot be entirely inclusive in an open group as others mentioned mental health triggers can be anything the brain makes an association with trauma, they have created the issue that bolsters the bootstrap brigade as trigger warnings are nearly always just things we socially determine to be difficult content which erases many people who have experienced being triggered by other types of content, and we've now had studies giving strong evidence that they're used to self-harm - some people when in a poor mental health state will deliberately look up content with trigger warnings to set themselves off further.