What triggers the problem analyzer?
Changing the type on a custom field can be fraught with difficulties. It's particularly challenging if you have data in your org which could be lost by changing the type. Different data types also have various restrictions on how and when the type can be changed and trying to deploy these changes can cause your deployment to fail.
How does Gearset help?
Given the challenges around deploying this type of change, Gearset will warn you when a deployment would change a field's type. You can then decide if you would like to proceed with deploying the change.