Likely either a short or a bad gfci. Unplug, disconnect equipment (heater, pump, blower, ozone, etc) one at a time, then plug in to isolate the fault.
Cord-end GFCIs are notorious for problems. If you can, get rid of it.
Any water on electrical components can cause a short and gfci trip, and it's not uncommon to get something wet when filling.
Problems are more likely after a fresh fill due to extended run time of equipment to get it heated up as well as any issues that developed while drained (freeze damage, rodents nesting, etc.). Also, improper fill can cause damage in some spas.
Pics of equipment area, circuit board, and wiring diagram may be helpful.