Yes, your hypothesis is very plausible.
How much do you weigh? How heavy of a load were you carrying?
Generally speaking, the lower the tire pressure the lower the efficiency (transfer of motor output to tire to road). Adding weight and not upping the pressure is equal to reducing the pressure at the same weight. So, as a 190lb guy, I cannot ride my 115lb GF's board (unless I add 20 PSI to it).
That said, if it didn't give you pushback, it could also be (in addition to the tire pressure) your feet are too wide on the board, or you slowed down and your foot wasn't on both sensors.
If your feet are too wide on the board, it makes it harder for the board to give you pushback (imagine trying to elevate a kid on a teeter-totter - if the kid is in the middle, no problem, if he's at the far end, difficult). When it does try to give pushback, it might overload and nosedive.