Google Sand box considered time frame or interval in which search engines find probation newly registered domain names, try to get quick rankings in google optimization by performing some illegal techniques. For some interval of time, they are getting desired results, but immediately lost time.
Google send some new site and old site in send box, it does not mean that your site has been penalized. its a normally process. google take all new domain under observe to check site and if google find that some site make a spam than take this site under observation,during this particular time website ranking may be go down. so don't worry sand box is not a harmful for your site.