Start with an observable trigger. Widgets, notifications, and links can open their app after an accidental interaction; that is different from an app randomly launching with no cause.
Check Back Tap first. In Settings > Accessibility > Touch > Back Tap, check whether Double Tap or Triple Tap is assigned to an app or a Shortcut. A case, a pocket, or a firm grip can trigger it without you noticing, and it looks exactly like an app opening on its own.
Check for ghost touches. A screen protector with an air bubble, a cracked screen, moisture on the display, or a case pressing on the edges can register phantom taps that open whatever is under them. Remove the case and screen protector and watch whether it still happens.
Check Shortcuts Automations. Open the Shortcuts app, go to Automation, and look for anything that runs an app when you arrive somewhere, connect to Wi-Fi, open another app, or at a set time. A personal automation can launch an app without a manual tap, and it is easy to forget you set one up.
Rule out a stuck notification or Live Activity. Tapping a notification banner, a Dynamic Island item, or a Live Activity opens its app; if your phone is in a pocket or bag, that tap can happen without you seeing it.
If none of that applies, restart, then update. Restart the iPhone, then check for and install any pending iOS update and any update for the specific app. A crash-and-relaunch loop in a buggy app version can look identical to a self-opening app, and Apple and app developers regularly patch this kind of issue.
Still happening after all of that? Note exactly which app, what you were doing on the phone at the time, and whether it is one specific app or several. That detail is what Apple Support or the app's own support team will need if the cause turns out to be hardware or a deeper software fault.