Alot of people think that parts are only triggered to front by external things or experiences, such as certain situations, people or environments, but that isn't true. Parts aren't only triggered to front by external things/experiences, internal things/experiences can also trigger a part to front.
Disclaimer: This post may not be entirely accurate, since I mostly based this off of our personal experiences. Don't see this post as 100% reliable. Feel free to correct me or share information I missed.
Internal front triggers (non exhaustive list):
Thoughts (eg. intrusive thoughts, daydreams)
Sensations (eg. hunger, fatigue)
Emotions (eg. panic, anger)
Health (eg. flare-up, fever)
Internal conflict between parts (eg. parts not agreeing on what happened in the past)
These potential triggers can be a result of external experiences, but that isn't always the case. For example: panic can be caused by a trauma trigger, but it can also be caused by an anxiety disorder or another disorder.
It's also important to note that there isn't a clear line between internal and external experiences.
Examples of switches (non exhaustive list):
The system dreamed about something that reminded them of a past trauma. When they wake up, a trauma holder is in front.
The system has alot of intrusive thoughts and feel scared or guilty because of that. Now an emotional protector is in front to stop them from spiralling.
The system feels very hungry. Now a caretaker is in front to make sure they eat well.
The system feels very angry. Now an anger holder is in front, so the other parts won't be affected by the anger later on.
The system has a fever. Now a symptom holder is in front to manage the symptoms.
Two parts are fighting. Now a gatekeeper is in front to temporarily separate them.
These examples are just examples. Each system will react differently to the same potential trigger. Some will switch to a caretaker, some will switch to a trauma holder, some will switch to a symptom holder, etc. Some won't switch at all.
We've only fairly recently figured this out ourselves, so I wanted to share this. I hope this helped some of you!