that has never happned to me. i reuse mine too. maybe you have holes in the bottom. its not a problem unless youre root bound
I got the same problem twice in a 3 gal smart pot after about 1 month. There are no holes on the bottom the roots just grow right through. The roots are not exposed to light is this bad??
Check out this thread http://forum.grasscity.com/organic-growing/1151159-noobwannbs-self-wicking-smartpot-ghetto-setup.html. Roots coming from the bottom of the pot is what we absolutely expect for a style of growing called Smart SIPs. If they bother you to see them, feel free to snip them off, it wont harm a thing if the plant is healthy. Some people say it shocks the plant but this myth has been proven wrong time and time again through out the thread I linked.
Its called "air pruning" and it takes air circulation to cause. So if air cannot reach the bottom of your cloth pots...........