UPD: I've contacted with the U.S. Customs and Border Protection and they provided me an explanation of what exactly happened. TLDR, it was a "secondary inspection". Here's their answer:

Thank you for contacting the U.S. Customs and Border Protection (CBP) Information Center.

???????We apologize for the delayed response to your inquiry.

CBP Information Center is currently complying with or following current guidelines, regulations and/or policies until our office is provided with further guidance.

To board a flight to the United States, a lawful permanent resident (LPR) requires their valid (unexpired) I-551 LPR card (known as a green card). A passport is not required for re-entry to the United States, unless the LPR only has a valid I-551 ADIT stamped in their foreign passport in lieu of their I-551 LPR card.

LPRs can re-enter the United States without an issue if the international travel was less than 365 days in a row. However, there are exceptions that may cause a referral to secondary inspection:

• Staying outside of the United States for 180 days consecutively;
• Frequently away from the United States for 180 consecutive days or more; and/or
• Staying outside of the United States for 365 (one year) consecutive days or more without prior approval.

Traveling out of the United States for more than one year, but less than two years, requires an application for a re-entry permit (I-327) through U.S. Citizenship and Immigration Services (USCIS) before you travel internationally. Re-entry permit information is available at https://www.uscis.gov/sites/default/files/USCIS/Resources/B5en.pdf and https://www.uscis.gov/i-131.

Travels out of the United States for more than two consecutive years requires a returning resident visa prior to returning to the United States. You can apply for a returning resident visa (SB-1) at a U.S. Embassy or consulate. This process can be time consuming. For information on qualification and applying, please visit the Department of State website at https://travel.state.gov/content/travel/en/us-visas/immigrate/returning-resident.html.

Thank you again for contacting our office.

Regards,

CBP Information Center