How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?

(N/A) Ionization enthalpies generally increase across the series due to the continuous filling of inner $d$-orbitals,which increases the effective nuclear charge.
The irregular variations are primarily due to the extra stability of specific electronic configurations like $d^{0}$,$d^{5}$,and $d^{10}$.
For the first ionization enthalpy,$Cr$ $(3d^{5} 4s^{1})$ has a relatively low value because removing one electron leads to a stable $3d^{5}$ configuration. Conversely,$Zn$ $(3d^{10} 4s^{2})$ has a high value because the electron is removed from a stable,fully-filled $4s$ orbital.
Second ionization enthalpies are generally higher than the first. Elements like $Cr$ and $Cu$ show exceptionally high second ionization enthalpies because their $M^{+}$ ions possess stable configurations ($Cr^{+}: 3d^{5}$ and $Cu^{+}: 3d^{10}$),making the removal of a second electron energetically demanding.