Outer Limits 4x4 Board

Went wheeling in the sand last week and just before hitting the rough stuff i dropped the tyre pressures down to 20psi.... Had a great day out and on the way home stopped off and added air at a garrage (back up to 40psi) BUT!
yesterday troopy was even harder to handle than usual so i checked tyre pressures and something weird had happened, the back wheels were running at 22psi right rear, 19psi left rear, 20psi left front AND 15psi front right tyre
WTF is going on.... I know i re inflated them on the way home and cant for the life of me work out how its happened.. Anyone ever had the same thing?
by the way the tyres are bfg AT, tubeless.

Sand in the beads maybe???????????????????

I agree with Coogs, most likely to be sand in the beads, not that uncommon. Depending on what you are doing but I as a generalisation do not recommend dropping pressures below 25 psi on bigger vehicles for this reason. At 25 you still get good elongation of the footprint, but reduce the chance of getting shit in the bead or rolling the tyre off the bead. If it is a lighter vehicle you can go lower.....

Happens to me almost everytime I go offroading. The local tyre place has taken pity on me now and clean my beads for $5 a tyre.