Funny you should ask
I have pondering something similiar in my own games. It seems that one of the worst times for my players is 7th-8th level. I've seen more player deaths at this point than any other level. This is the theory that I came up with, I call it "The Seventh Level Split."
1st level is a dangerous time for PCs. They usually don't have much magic, many HP and their bonuses and skill ranks aren't too high. Usually, though, a DM keeps things pretty much at a level where the party can make it through. In my 20-something years of gaming, I've seen relatively few 1st level PC deaths.
The fact is -in D&D- even low-level characters are pretty powerful. By the time they reach 2nd-3rd level, they are significantly stronger than the average peasant/NPC. The characters have also usually accrued a few magic items by this time as well; it's all good.
As a result of this "relative power level," most combat challenges the PCs face can be addressed through main force: A cry of "HOODY HOO!" and "I WASTE IT WITH MY CROSSBOW!" can be heard and -next thing you know- problem solved.
Now, I'm not saying this is broken. 3rd level PCs shouldn't be facing too many foes with "save or die" attacks, etc. Nor am I saying that DMs only throw "stand up fights" at these parties. What I am saying is that most "monsters" of an appropriate CR for these groups are usually just bigger, stronger versions of what they faced at 1st. ie: bugbears instead of goblins, an ogre instead of some orcs, etc.
What parties should be doing (and I'm sure many do) during this time is learn to work together. They should be developing successful group tactics and strategies, getting to know what the other characters' strengths and weaknesses are, talking to each other about better ways to meet future challenges, etc. All too often it seems, the party seems to adopt a "It ain't broke so don't fix it!" approach and just count on new Feats & Items & Spells & more HP to see them through; and the truth of it is, this DOES work.
Up to a point, that is.
What I have encountered -and heard many stories of from other gamers- is that along about 7th level or so, a "party of individuals" just attacking for maximum damage isn't always enough. It seems that at this point (or near it) is when the teams keep winning and the 4 or 5 "solo adventurers traveling together" get into trouble. The cleric isn't holding back waiting to heal, he wants to get his licks in. The fighters charge into melee range right away, not giving the wizard a chance to cast a fireball in order to soften the enemy(s) up, etc. The party has reached a point where they need to fight smarter, not just stronger. Many groups don't make this connection and pay the price.
On a side note: Amusingly, the one character type I've seen semi-consistently played in this "team player" style; which -imho- gives better odds for reaching more powerful levels is the Bard; which rarely seems to get played. (shrug)
Anyway, this post has gotten fairly long so I'll wrap up by saying that I think 7th level or so is fairly deadly for the reasons listed above. The truth is though, the I think these "Splits" occur at other points in the level progression and when people talk about TPKs at __th level, it is often very probably for similiar reasons. The party's tactics have to adapt to new challenges; getting another d6 damage on your Sneak Attack, etc. isn't going to be enough. That's my theory based on my experience. YMMV
(edited for grammar)
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