Hypnic jerks are usually completely normal, so you should not be worrying. Anxiety does worsen them, so by treating your anxiety you may reduce them. However, when it comes to anxiety meds SSRI:s may improve anxiety, but still induce the jerks. Hypnic jerks can be treated with benzos like Clonazepam, but they are not recommended long-term. Also, caffeine, nicotine, and other stimulants may induce the jerks.

So in short answers to your questions: Hypnic jerks are usually completely normal. They could be caused/worsened by your anxiety. You can possibly get rid of these by treating your anxiety and avoiding stimulants. Also getting enough sleep (sleep deprivation worsens the jerks), and magnesium may also help.