Its just insinct, IT THINKS YOUR ITS MOMMA!! LOL, MY SISTER HAD A CAT THAT WOULD SUCK ON BLANKETS, YOUR EAT, ANYTHING THAT WOULD CALM HIM, ITS KINDA LIKE WHEN A CHILD SUCKS HIS THUMB. MY SISTERS CAT DID THAT HIS WHOLE LIFE.
Kittens that are taken away too early tend to not mature as well, and want to find "mom" again so they can suckle. It makes them feel secure. So they will suckle, and knead, and purr, because suckling and kneading makes them think of happy times when they were small, and warm, and protected, and perhaps you will give them something like "mom" used to give.