Even a steady ascent can be followed by a rapid decline. Peter Chou, chief executive officer of HTC, knows that only too well. Earnings at his Taiwanese mobile phone company grew more than fivefold in the six years before 2011. Yet on Monday when HTC reported results for the fourth quarter of 2012, it announced the lowest quarterly profit since 2004. That puts full-year earnings on track for a three-quarters drop from their peak in 2011. The question is why HTC’s shares have rallied by a half over the past two months.