(covers information from several alternate timelines)
Kes begins to travel back in time due to her cells existing in a state of biotemporal flux
Biotemporal flux was a state in which an organism's cells could exist as a result of exposure to chroniton particles.
In 2379, when she was exposed to the biotemporal field of the bio-temporal chamber created by The Doctor in an attempt to extend her lifespan, Kes's cells were put into a state of biotemporal flux due the reactivation of dormant chroniton particles in her cells, causing her to experience a series of backward time jumps, essentially living her life in reverse. When Kes arrived in 2373 of an alternate timeline, and explained what was happening to the crew of Voyager, The Doctor used a bio-temporal chamber to expose Kes to a precisely modulated field of antichronitons in an attempt to purge her system of the chronitons, and bring her back into temporal sync with the rest of the crew. When Kes was exposed to the antichronitons, she experienced a series of jumps, all the way back to her conception, after which she returned to 2373, regaining all of her memories of her life up to that point. (VOY: "Before and After")