KABUL, Afghanistan — Afghan observers and Western officials are interpreting the forced resignations of Afghanistan’s two top security officials as another worrying sign of President Hamid Karzai’s increasingly impulsive decision making and deepening isolation from his backers, both within Afghanistan and abroad. The two men who resigned over the weekend, Interior Minister Hanif Atmar and the intelligence director, Amrullah Saleh, had strong relationships with American and British officials and were seen as being among the most competent of his cabinet members, said several Western officials in Kabul. Mr. Saleh, in particular, had built an…