Structures generation

[TobiBlackmore]

So I made this topic before: http://forums.technicpack.net/threads/mysterious-tree.53118/

And now I was exploring to collect hexical flowers, and after I mined one, it dropped nothing, so naturally, I thought I was having lag. So I waited, and it brought me back to the flower, untouched, and it changed into a stone brick block. Suddenly, I was below a floor of stone brick blocks. Basically, this spawned on my head;

I was on the left, below the wall. It spawned away my hexxit flower...

Now I'll give credit to this one that it might not have been generated in the chunks yet, but it happened to me before, at a place close to my own house I've been at for HOURS.

Anyone know how this happens?