We are seeing something unusual occur with one of our process models. When

Check the start event properties, does it have any triggers setup?

@davidc If you haven't made a search in the forum already, I would like to suggest you to go through following links:

https://forum.appian.com/suite/tempo/entry/e-182367
https://forum.appian.com/suite/tempo/entry/e-108038

The posts at above links gives you an idea of how you can control the behavior as per your requirements but to the best of my knowledge preventing the model from triggering the instance isn't possible.

What I generally prefer in this case is following.

To have an XOR gateway after the start event and configure a constant (Boolean type) in the XOR gateway. While publishing the process model I make sure the constant is set to false which will redirect the flow of the process instance to end event.

Note: The only constraint in this that one need to check the value of constant each time one publish the process model.

Now to remove this dependency, I generally put my logic in the sub-process and publish the sub-process process model and the main process (which is having a timer event) in never published.

We have used the method as aayusha suggest, but an alternative is to open the timer event advanced settings and choose your Boolean constant in an expression under Timer Conditions, causing the model to only fire if the constant is true(). This will result in the same behavior as the XOR configuration.