The rot started when the families owning the title sold out to Conrad Black. When the latter's businesses unwound, the Telegraph passed to the Barclay brothers. One can at least say of Black that he knew the newspaper business. Although the Barclays had been involved in newspaper ventures in the past (e.g. The European, whose objective reporting on continental matters they switched to an anti-EU stance) but their understanding of journalism was clearly lacking. Their management of the Telegraph has been marked by a series of gimmicky appointments, chronicled by Private Eye, which have improved neither its profitability nor, it seems, the morale of the journalists who remain. Now the most senior of them has jumped ship. On the Our Democracy web pages, Peter Oborne explains why.
His article deserves reading in full. He outlines the status of the paper when he joined and the decline in standards under a succession of editors (or "heads of content"), leading to blunders which would be ridiculed in a red-top tabloid. Worse, as the country's revulsion against big business corruption tops the domestic headlines, is the way that the demands of big advertisers has been seen to distort its presentation of the news.
See also http://order-order.com/2015/02/18/leaked-memo-shows-telegraph-offered-sony-support-across-editorial/