Single Strand Knots or Fairy Knots as some people may call them are caused by dryness, lack of moisture, and split ends. When dry hair rubs against each other the weaker strand tends to coil back forming a knot. Eventually that knot will break - causing more damage because that breakage will sometime continue up the hair shaft.
Under a microscope, a single strand knot looks like the pic.
A few ways to prevent single strand knots:
Shampoo or Co-Wash: If you are shampooing or co-washing do it in sections. Part the hair into 4 to 8 sections. Cleansing your hair in sections will help to prevent tangles and matting. When finish with one section twist it, clip it or put it in a Bantu knot and move on to the next section. When all sections are done, rinse with the hair still in sections; this will result in even better detangling when you get ready to apply your leave-in and style.
Detangle: You can either detangle with your fingers only. Or, you can start off detangling with your fingers and then transition to comb. Using your fingers help you feel for knots and tangles that the comb may miss. Proper detangling will help prevent the knots from breaking and curling back on themselves. Make sure you use conditioner on your hair when detangling because the conditioner will provide the slip you need to gently detangle your tresses.
Hot oil treatments or oil rinses: help invigorate your hair. Hot oil treatments and oil rinses penetrates the hair shaft, fills in those split ends, and combats frizz. You can either use a store bought hot oil treatment or you can use a natural oil such as coconut oil, jojoba oil, olive oil, argan oil (only a tbs full), or whatever your favorite oil may be.
Green tea rinses: Green tea contains panthenol and Vitamins C and E. Panthenol strengthens and softens the hair and protects the hair from split ends while the vitamins are working to restore and protect the hair strands. Prepare 2 cups of green tea, shampoo hair or co-wash, rinse, pour green tea over the hair, massage into the scalp and work into the ends, cover with a plastic cap, sit under a dryer or place a heat cap on your head for 30 minutes, rinse, condition, then continue with your normal regimen.
Deep conditioning: on a weekly basis also helps because deep conditioning treatments are like massages to the scalp. The deep conditioner penetrates the hair shaft, adds moisture, repairs the damage, and leaves the hair healthy and shiny. The heat, whether you're using a dryer- heat cap - or steamer, is allowing the conditioner to penetrate those hair follicles. The heat is like a masseuse. It is stimulating the blood flow and increasing the oxygen levels. This helps to condition the scalp and promotes healthy hair growth.
Seal in moisture: after shampooing, conditioning, and/or co-washing make sure you seal in the moisture by applying a natural oil, such as olive oil or argan oil, to your tresses.
Trim as needed: Trim your ends as needed. If they appear jagged or split, a good trim will help minimize those split ends. Check your ends on wash day to see if they're still in good condition. If they're looking a bit jagged, trim them and follow with an oil to seal.
Protective styles: If you are transitioning, protective styles are a great way to protect your ends. Protective styles are low maintenance and they are a great way to give your hair a break.
Night protection: Wrap your hair in your favorite scarf or cover with your favorite bonnet. Satin/silk pillowcases help maintain the moisture balance in your hair. The hair is able to slide without tension or snagging while you sleep. Even though satin and silk are highly recommended in the natural hair community, you have to use what works best for your hair.