Normally, the AVR hardware clears the global interrupt flag (in SREG) before entering an interrupt. This means other interrupts are disabled inside the running handler until the handler finishes and exits.
The RETI instruction is the normal function epilogue for an interrupt handler, and thus re-enables interrupts (just like executing an SEI instruction).
For this reason, normally, interrupt handlers can not be nested.
Search ISR(XXX_vect, ISR_NOBLOCK) for a way around this.