We know that all Valyrian sorcery was based on blood and fire, which along with iron happen to be the key ingredients of the Iron Throne. All of which makes me wonder whether Aegon wrought some kind of magic upon the throne so that all those who usurped it died before their time.
Maegor: Actually died on it
Aegon II and/or Rhaenyra: Poisoned and consumed by a dragon.
Robert: Killed by a pig
Joffrey: Poisoned at his own wedding
Tommen & Myrcella: We know from the prophecy (and show) they're doomed.
It's not as if anyone who takes the throne legitimately ldies of old age in their bed (Aenys, Aegon V...), but the fact that every single usurper dies unnaturally seems too much of a coincidence.
The throne's also cruel to those Kings who rule poorly, Aerys 'King Scab' being the best example, but Viserys I's slicing his hand open at the moment of dealing poorly with the succession also seems like a sign.