Is there a way to reliably set rooms on fire?

The best way to cause a fire or radroach infestation is to rush a room several times in a row. Beyond that, there's no way to ensure you always one of those events vs. the other event.

For reference:

• Having an active radio room increases the frequency of deathclaw and raider attacks.
• Opening your vault door increases the frequency of deathclaw and molerat attacks.
• Building rooms increases the frequency of molerat attacks.

You can build a room isolated from the rest of your shelter to do repeat rush.

If you do not get a fire, run your dweller out of the room and wait for the incident to die down on its own. Because it is isolated, it'll just end without you needing to end it.

This method can prevent excess happiness loss in rush failures.