Why does adding one more extension cable turn the MCB on again?
The additional extension cord adds more resistance, reducing the inrush current.
The inrush current required to trip a 20A breaker instantly can be as much as 300A, though it's more likely to be about 150A. This means the resistance of the whole system is less than 0.8 ohms.  An extra hundred milliohms, as would be added by a 20 foot 14 AWG extension cord, would make a difference of tens of amps of inrush current.