Background: Gulbuddin Hekmatyar was born in 1947 in Imam Sahib District of the Kunduz province, northern Afghanistan, a member of the Kharoti tribe of the Ghilji Pashtun. His father, Ghulam Qader, who migrated to Kunduz, is originally from the center of Ghazni province. Afghan businessman and Kharoti tribal leader Gholam Serwar Nasher deemed Hekmatyar to be a bright young man and sent him to the Mahtab Qala military academy in 1968, but he was expelled due to his political views two years later. From 1970 to 1972, Hekmatyar attended Kabul University's engineering department.
Context: In May 2008, the Jamestown Foundation reported that after being "sidelined from Afghan politics" since the mid-1990s, Gulbuddin's HIG group has "recently reemerged as an aggressive militant group, claiming responsibility for many bloody attacks against Coalition forces [at the time, primarily the International Security Assistance Force and the administration of President Hamid Karzai." The re-emergence of this "experienced guerrilla strategist" comes at a propitious time for insurgency, following the killing of Taliban commander Mullah Dadullah, when some elements of the Taliban were becoming "disorganized and frustrated." HIG has claimed responsibility for and is thought to have at least assisted in a 27 April 2008 attempt on the life of President Karzai in Kabul that killed three Afghan citizens, including a member of parliament. Other attacks it is thought to be responsible for include the 2 January 2008 shooting down in the Laghman province of a helicopter containing foreign troops; the shooting and forcing down of a U.S. military helicopter in the Sarubi district of Kabul on 22 January; and blowing up a Kabul police vehicle in March 2008, killing 10 soldiers.  In interviews he has demanded "all foreign forces to leave immediately unconditionally." Offers by President Hamid Karzai to open talks with "opponents of the government" and hints that they would be offered official posts "such as deputy minister or head of department", are thought to be directed at Hekmatyar. It was reported in 2008 that Hekmatyar lived in an unknown location in southeastern Afghanistan, close to the Pakistani border. In 2008, he denied any links with the Taliban or al-Qaeda and was even considered for Prime Minister. Hekmatyar is now believed to shuttle between hideouts in Pakistan's mountainous tribal areas and in northeast Afghanistan.  In January 2010, he was still considered as one of the three main leaders of the Afghan insurgency. By then, he held out the possibility of negotiations with President Karzai and outlined a roadmap for political reconciliation. This contrasted with the views of Taliban leader Mullah Omar and allied insurgent chief Sirajuddin Haqqani, who refuse any talks with Kabul as long as foreign troops remain in the country, Hekmatyar appeared less reluctant.
Question: Why had they re-emerged?

Answer:
an aggressive militant group, claiming responsibility for many bloody attacks