From my rather extensive experience in the game, here is what I have seen that causes and prevents resurrection:
Causes:
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Wave Battles
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NPC Ratio: The closer the NPC

C ratio gets to 1:1, the higher likelihood of resurrections. At even 2:1, I expect a few resurrections in a weekend. At anything truly approaching 1:1, even a massively underscaled wave battle has a reasonable chance of causing resurrections.
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Mass Loss Groupings: This is a theory I learned from Battletech, but applies to Alliance as well. For the most part, combatants fight at close to 100% efficiency until the very moment they drop. As such, it is common for many to be fighting "on fumes" until suddenly a large swath are incapacitated at once. Resurrections ensue.
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Repeated Delivery Types/Carriers: There are enough defenses that the PC population of most games can face an entire wave battle of creatures swinging Drain or throwing Poison Sleeps (so many defenses to that it is crazy). But, if that is the main type of opponent for the weekend or multiple wave battles use that mechanic, eventually every hit is basically an unstoppable disable. That is "run for the wards" time.
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Modules
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Bad Scaling: Module scaling is significantly harder than weekend or wave battle scaling. Non-combat skills, time since reset, and player skill has a much higher influence on PC ability than in wave battles. Also, there is basically no safety net. In plenty of modules, the nearest help is more than five minutes away (round trip).
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Disorganization: Ever see a module team with 4 earth casters and 2 rogues? I have. Unless the module involves undead, it is usually referred to as a TPK. Teamwork is important, but balance is even more important. A poorly balanced module group is basically begging to hear a 15 minute long story.
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Random
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Dark Clothes at Night: Skulking has got to be the number one way to earn a resurrection. PCs don't see you when you are lying on the ground, but some NPC does and lands a Terminate on your back.
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Heroics: I forget what movie it is from, but there is a great quote: "Do you know the definition of a hero? It is someone that gets other people killed." Yep. Heroics are cool, but they very often get someone (possibly you) killed. You want to live longer? Don't be a hero and avoid heroes.
Prevents:
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Magic Items: As much as I hate to admit it, it is the elephant in the corner. The more you glow like a Christmas Tree, the less likely you are to die in any given situation.
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Rules Knowledge: Not rules lawyering, but rules knowledge. The game has a crazy number of effects. Knowing precisely what counters what and most efficiently is key to survival. Similarly, knowing exactly how dangerous an effect is also keeps you alive. For example, against a large group, Fear is rarely deadly. Don't waste resources removing it. Let the time run out. That Awaken may be much more essential to remove a Berserk from Super Beat Stick Man later.
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Buddy System: Better yet, team system. If you are always responsible for the whereabouts of one or more people and one or multiple people are responsible for your whereabouts, you are much less likely to die, unless large swaths of people are dying.
-MS