What are some possible causes of the BTC-e 'flash' crash? What circumstances made this possible? What entities or major factors could be the cause?
Flash crashes can and will occur in any market where technology (algos) determine trading.  Always tailed by real people panic selling (or panic buying).

Doesn't matter what market you are in.

There also the effect of futures trading (I know of a few futures/options houses that operate in the crypto-space).

There is really nothing you can do.  Worrying about it now (while it is happing is pointless). But to give you an answer (at least for litecoin pricing pressures) newer ASICs miners are 'supposedly' creating too much liquidity.

There are quotes from the old world of stock trading that ring true here (non-cited and paraphrased):

"The market is designed to make as many people look as stupid as possible..."

and 

"Do you want to look like a fool before the crash or after the crash..."

Seems to apply pretty well to the new and shiny crypto-currency markets too.

But there is another quote that is pretty famous and great to light up those of us enveloped in the darkness of nasty market moves.

"Be fearful when others are greedy and greedy when others are fearful..."

Here is the thing, when everything is mined to the point that it is simply not feasible to continue and coins are lost over time then you will find yourself in a deflationary situation where all of a sudden your coins (bitcoins and litecoins) are hit with supply-side pricing movements to the up-side that will pale in comparison to anything you have seen before. 

Since we are about half-way there, I would recommend relaxing and watching the fireworks because the low-hanging fruit is being consumed.  

Finally, I would venture a guess that we will see this supply-side anomaly begin to take shape in early 2017.