It isn't controlled.
That is absolutely true because in a free market if you are able to invest then we will be able to control bitcoin's price levels. At the same time our control will not sustain for long as anyone else will be having more investment than us to take control from us.
It is a free market and we all control it.
actually it is controlled. to a certain amount.. and done so by people. rich people with all the trading tools
it is impossible to arbitrage from one exchange to another near instantly for regular folk.
there are bitcoin confirmation delays
there are fiat bank wire delays
so knowing this. ask yourself
how come the different exchanges sheep follow each other up and down so fast.
the answer is that the exchanges themselves have mixed reserve they work with.
this is using probably the blockstream 'elements' system where only the exchanges get to arbitrage behind closed doors, instantly
5 years ago mtgox and btc-e had something similar where bitinstant was the arbitrator tool.
so while usual traders are depositing funds to make orders minutes later. the exchanges have already put their large orders in to play the markets and basically sheep follow each other near instantly and get to be first place inline for the best prices
But I still don't get it much, if you said that it's because the demand for bitcoin is on the increase, then why does the price fluctuate so rapidly and uncontrollably like never before? I think it must have a steadier growth than now, as all I can see from the minute-by-minute updates, the price changes every minute and the second before it was still high and one minute after it plummeted. 
