Whyyy is my basement toilet a bubbling mess?

Every 3ish months it happens; basement toilet starts bubbling/gurgling when someone’s having a shower upstairs. I’ve guessed it’s the massive tree out back growing into the pipes and have paid to have the main lines cleared whenever it happens..but every 3 months seems kind of high. No root carnage is left when the plumber is done, but all drainage is fine afterwards so …?? If it’s not roots, what’s happening?

Lines last cleared Dec.29 2025 and the issue has started again - April 4 2026. Tree out back is a 2+ storey silver maple. Problem persists through wi nter as well.