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.