Islamic conservatism, and radicalism, have long been popular in Yemen. The bin Laden clan is Yemeni, and Osama bin Laden still has some family there. In the late 1990s, the government, worried about Islamic radicals getting out of hand, and under pressure from the United States, shut down al Qaeda, and other Islamic radical organizations. But many Islamic radicals simply went underground. The Yemeni government has never been able to control the Yemeni tribes, and the tribes are the most enthusiastic about Islamic radicalism. If the tribes give Islamic radicals sanctuary, and this happened often, there was little the government could do about it.
Syria and Yemen have even been exchanging information on Islamic terrorism, as their two countries were often seen as convenient hiding places for Islamic radicals. There are still factions in Syria that can protect pet radicals. Up to a point. The same with the pro-terrorist tribes in Yemen. So both Syria and Yemen try to keep track of terrorists that move back and forth between the two countries. There have apparently been some prisoner swaps as well. The United States wants more access to this information, and some of the imprisoned terrorists, but that has proved difficult to accomplish. You can still trade favors with Islamic terrorists. A little protection here gives you a little immunity-from-attack there. Its a dangerous game, and Syria and Yemen are cooperating with each other in an attempt to increase their survival odds.
Many Moslem countries that once tolerated, or even supported, Islamic terrorism, have changed their minds. Sort of. Its a complicated situation. Take, for example, Syria and Yemen. Both nations have long been the home of many Islamic terrorists. For Syria, it was state policy. Islamic terrorists who hurt Syrias enemies, and did not attack Syria, were welcome to set up shop, openly or covertly, in Syria. But the American invasion of Iraq fired up Islamic radicals, especially the ones in Syria, to such an extent that the government feared for its own existence. Thus the recent willingness by Syria to cooperate in cracking down on Islamic terrorism.