For the most up to date information please keep a close eye on your tracking number.
Packages can be returned to Japan for a variety of reasons. If the package reached customs in your country, there is an extremely rare chance that customs could refuse it for some reason. Again, this is extremely rare.
More common reasons your package would be returned would be due to an undeliverable address, package not claimed at post office or package not claimed at customs. Sometimes the post office will send you a notice to collect the package at the post office or customs office. If you do not do that within the specified time (usually 3 days to 2 weeks) the package can be returned.
For more detailed information on your countries unclaimed returns, please speak with your local post office.