Depending on the type of sensor they use, it might *have* to be calibrated every time, because some of them have a different "zero" value everytime they are powered on. (like the type used in a THUN BB sensor).
Without the calibration, then depending on sensitivity settings and how far from actual zero the "zero" starts out as, it's possible the bike could just take off on it's own simply from the difference between the last calibration of zero and the zero the sensor has at that power on.
So that is probably why it happens every power on.
What I would do is have a calibration routine that flashes the whole LCD, every icon and segment on it, to make sure to catch the rider's attention, until it's done. If there's any way to display text on there it could also say what's going on, or it could have a word-icon for "calibrating" that *doesn't* flash while the rest of the display does.
Then the actual calibration process would fault if it's out by too far (it could be pretty far off with feet on pedals), and simply keep retrying (and flashing the display) until it passes, or if the display can show text, it could say "calibrating, keep feet off pedals until done", and then display "done, ready to ride" whenever it is complete.