The most common cause is due to short/leakage to Earth and the resistance is high enough not to cause tripping (below RCD sensitivity) during normal everyday use, but easily trip during slight lightning surges.

Get an electrician to trace and fix the short/leakage. It could be from appliance, loose wiring/fitting, damaged cable etc. Beware and avoid lazy 'solution' such as disconnecting earth wire or RCD with lower sensitivity (higher tripping current).

No. If the leakage is from Live wire To Earth, the leakage is too low or it will already trip even during normal day. If the leakage is from Neutral or appliance (that don't have 2P switch/breaker), turning off MCB won't help as the Neutral wiring still connected to the busbar and only small proportion of the current will flow through the leakage.

It is actually easy to confirm if you have short/leakage to ground but I'm totally against anyone without sufficient experience and knowledge to DIY and open the DB themself.

There is no leakage per se, if there is a leakage said RCD alr tripped.

Sometimes its due to RCD from the 90s
a) aging >30 years tolerance all lari and/or
b) no "anti nuisance trip" filter technology

Replacing a 90s' RCD to a current model ABB solved nuisance trips for me.
ABB or Hager 30mA RCDs are the best you can get (easily) in MY market.