it happens with some strains with high THC and little CBD. Most but not all sativas are like that. Some indicas are also actually like that. THC is an hallucinogen and also a stimulant, CBD is a downer. Hallucinogens can be classified as deliriants, dissociatives and psychedelics. Cannabis/THC can be mildly psychedelic in high doses, but other cannabanoids can block THC and make you feel tired, lazy, and mellow. THC will make you feel up, high, energized. That triggers adrenaline to go, and a panic attack is caused by a flood of adrenaline. THC is mainly a dissociative and so when you feel adrenaline, you start to panic, and the dissociative effect amplifies that feeling and sometimes feels like it won't go away. I usually just got with it because I know it's just a reaction and will only last a couple of hours, but it can be very intense sometimes because of the dissociative effects.