Does depression always come with a "trigger" event or can depression sometimes appear with no apparent reason?

It can be either, some people experience depression without any reason or trigger, unfortunately it can be something that just occurs.

Depression can absolutely develop with no clear "trigger." It is widely accepted that susceptibility to depression is passed on genetically. As a chemical imbalance in the brain, it does not require a "trigger" event to manifest.

There is often time a trigger event that over time gets lost due to our inattention to it. Especially for those who are chronically depressed, the beginning is usually very muddled.

From personal experience, depression can definitely appear with no apparent reason. You don’t ever have to have a ‘reason’ to feel depressed and while certain events such as a death or bullying can cause depression, it’s totally possible that depression can seem to come out of nowhere

Well if the reason is not apparent to us, it does not mean that there is no reason. If one think he or she is not triggered at a certain moment, also does not mean that something haven`t pop up from their subconscious mind. Sometimes, or better, most of the time when we are depressed it is a sign that we need to have a rest. From all situations, people, impressions, our feelings and our projections and reflections and perceptions in and on the world we live in. It mostly seems that depression comes out of nowhere but in my own experience, it often arrived after accumulated unexpressed emotions and words. With or without clear reason, depression is like automatic body-soul alarm that says "stop, overview, relax, let it go..." So we are forced to stop and to look and listen. To find the reason. And will and strength to solve the puzzles.

Depends what you mean by a ‘trigger’. If you mean, some sort of underlying cause, then yes, but if you mean an event or trauma, then no. Depression is, to the best of my understanding, a mental response to something. What that something is depends on the circumstances and the individual. Some people have a congenital predisposition to depression, some people inadvertently condition themselves into depression-inducing thought patterns, some people go through traumatic events that cause depression. If depression is caused through something congenital, such as some kind of flaw in your brain’s ability to deal with various hormones, there is no external cause. I fall into this category - I’ve been told that for someone who has dealt with depression their entire life, I’m remarkably resilient and have a surprisingly positive attitude. So despite having had a good mindset with no major traumas or anything like that for my entire life, something out-of-whack in my system causes me to have depression. There isn’t any ‘trigger,’ beyond having a few trick genes.