One would guess there's a speed sensor that isn't telling the controller how fast the thing is going so the controller assumes the belt isn't moving and cranks the speed up. Rinse and repeat until the thing is running at full speed.
Fault is probably in the speed sensor (if separate from the motor) or the controller.
Failing that, the output of the speed controller may have failed to permanently "on"
These may help
http://silentdevelopment.blogspot.fr/2017/01/reebok-i-run-disasembly-maintenance.html
https://www.manualslib.com/r/reebok+i-run.html
acp, June 2017
