The induced emf in a L-R circuit is maximum :

${\mathrm{\epsilon }}_{\mathrm{L}}=-\mathrm{L}\frac{\mathrm{dI}}{\mathrm{dt}}$

The induced emf will be maximum when the rate of change of current is maximum.
Since at the time of switch off the rate of change of current is maximum, hence the induced emf is maximum and the resistance of the circuit is also maximum as,

$\mathrm{Z}=\frac{\mathrm{V}}{\mathrm{I}}$