When the I2C bus drives SDA2 low, SDA1 follows after a certain delay in the receive path. The output low is the buffered output of VOL1 = 0.75 V, which is sufficiently low to be detected by Schmitt-trigger inputs with a minimum input-low voltage of VIL = 0.9 V at 3 V supply levels.
When SDA2 is released, its voltage potential increases towards VCC2 following the time-constant formed by RPU2 and Cbus. After the receive delay, SDA1 is released and also rises towards VCC1, following the time-constant RPU1 × Cnode. Because of the significant lower time-constant, SDA1 may reach VCC1 before SDA2 reaches VCC2 potential.