When forward voltage is applied across anode and cathode of an SCR, Junction $J_1 $, $J_2$ are forward biased but junction $J_3$ is reverse biased and act as a capacitor. Then the charging current is given by

$$i = \frac{d_q}{d_t}$$

Where q is the charge and it is given by $$q = c_j v_s$$

$$i = \frac{d}{d_t} (c_j v_s)$$

$$i = c_j \frac{dv_S}{d_t} +v_s \frac {dc_j}{d_t}$$

but $c_j$ is almost constant

$$\therefore i = c_j \frac{dv_s}{d_t}$$

If the rate of rise of forward voltage increases, the charge current i also increases, this current turns ON the SCR even when the gate current is zero.

The turn ON is called $d_v/d_t$ turn ON and it leads to false operation of the thyristor.Hence it is necessary to protect SCR from $d_v/d_t$ turn ON