They could think that you are abandoning your LPR status in the US.
The legal permanent resident status is jeopardized when certain conditions are met:
"Maintaining Permanent Residence
Maintaining Permanent Residence You may lose your permanent residence status if you commit an act that makes you removable from the United States under the law in section 237 of the Immigration and Nationality Act. If you commit such an act, you may be brought before the immigration courts to determine your right to remain a Permanent Resident.
You may be found to have abandoned your permanent resident status if you:
* Move to another country intending to live there permanently. * Remain outside of the US for more than one year without obtaining a reentry permit or returning resident visa. However in determining whether your status has been abandoned any length of absence from the US may be considered, even if it is less than one year.
* Remain outside of the US for more than two years after issuance of a reentry permit without obtaining a returning resident visa. However in determining whether your status has been abandoned any length of absence from the US may be considered, even if it is less than one year.
* Fail to file income tax returns while living outside of the US for any period.
* Declare yourself a “nonimmigrant” on your tax returns." (taken from:
http://www.uscis.gov/portal/site/uscis/men...00045f3d6a1RCRD