Why isn't it a relevant question to understand the motivations of your enemy? Unless you're saying that Da'esh has no motivation and is just doing things randomly.

It may be that they want an increased commitment of troops because they believe it will lead to the prophesied climactic battle that ends with Allah destroying the unbelievers.

It may be, sure, but it may also be that they want everyone to think that about them, in order to produce fear. Appearing totally crazy is often a good strategy in warfare.