In the simplest terms possible, because this is a reasonably complex scam:
Adoboli held stock A and believed the price of stock A was due to rise.
Part of UBS's risk control system (designed with the sole intention to limit downside risk i.e. large trading losses) was to require all trades to be hedged to some degree.
Hedging a trade involves taking the opposite position in stock B that you took in stock A. Therefore, if stock A falls when you predicted a rise, the majority of your losses will be recouped by your short position in stock B.
The first part of Adoboli's fraud was to bypass these risk control measures to such an extent that he created fictitious positions in stock B. In other worse, he was running naked positions and was completely unhedged but was reporting to his superiors that his large loss making positions were completely hedged (often siphoning funds from his umbrella account to credit the shortfall).
The second part was to falsify his daily Profit & Loss statements. All trading desks no matter how large or small will report P&L on a daily basis (at least). As a trader you are ultimately responsible for your own P&L and its accurate reporting. Adoboli used his experience in trade support and settlement to create fictional (but believable) daily P&L prints, thus enabling him to run his loss making positions under the radar in the hope that they would come good. Normally such positions would be flagged on market close and automatically closed out but because the status of these positions was being falsified, there was no traffic light that ordered trading to stop.
The final part of the scam was to create an umbrella account, into which trades that could not be legitimately booked due to their dubious accountancy and risky nature were booked. These traders were winners, they made money. But they could not be reported because doing so would alert Adoboli's colleagues to his questionable trading methodologies. The umbrella account was certainly used by some of Adoboli's colleagues though, but it is wrong to say that he was the scapegoat. He created the account, used the account far more than anyone else and ultimately ran the account into the ground. Imagine if I robbed a bank for a million, gave you a grand on a night out and then when I eventually got caught I tried to finger you as my accomplice simply because you took a grand from me on a night out without asking any questions. This is basically the extent of involvement that Adoboli;s colleagues had.
The reason Adoboli got caught was because he got it wrong and he got unlucky. He got it wrong because he placed the wrong bet, he got unlucky because UBS were tightening their risk control measures in line with a wider risk off sentiment throughout the bank and Adoboli's positions got flagged.
Anyway Adoboli got it wrong because he called the market wrong. He bought when he should have sold, and bought more when he should have gone flat. He betted, he didn't trade. Despite the aforementioned article lauding Adoboli as some sort of maverick trader, he was just a gambler who dug himself into a deeper hole as he tried to recover his losses. He was no scapegoat. To call him such would be to discredit the hard work and moderate level of intelligence that it took to run such a scam for almost 3 years at a bank that was constantly tightening its risk.
And for those who said he is calm and will take it on the chin, he was crying like a little girl in the dock about how he "did it all for the bank he loved" whilst at the same time trying to finger as many of his colleagues as possible.