I believe in case of IOS it is either due to low RAM or ARC messing up. RAM issue is related to the idea of mobile devices not having a separate RAM drive, meaning it partitions its integrated storage to possess a modifiable RAM pool. Conclusion: If you have low space overall, you dont have much RAM available, meaning your game will crash more often.
ARC has to do with something I lazily call rps (requests/references per second). Essentially, and please correct me if I am wrong (This is a call to any professional IOS developers), it is a system that manages memory within an application (While RAM concerns itself with the entire device and is managed by the IOS itself). Usually, ARC does well enough, releasing any unused memory, yet a bunch of conflicting requests done in a rapid succession can cause it to mess up and shutdown the entire application (As it runs out of memory). This is generally observed when you try casting Tartarus with Aridne (many troops) or Casting Tartarus while spamming spearmen through the thread portal.
Talking about Aridane, @CaptainMorgan, it may be a good idea to have a hardware limit on how many units Tartarus affects and/or some sort of delay between the effects to quell the issue. It is very frustrating to use gems, trophies, vps and furies when the game crashes in front of a gate