We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with H∥c on an underdoped Ba0.75K0.25Fe2As2 pnictide single crystal, with superconducting transition temperature Tc=28 K. The second magnetization peak (SMP) has been observed at temperatures below Tc/2 and vanished at higher temperatures. The observed behavior of the SMP has been studied by measuring the magnetic field dependence of relaxation rate R(H) and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting H−T phase diagram is obtained. The critical current density (Jc) was estimated using Bean's model and found to be ∼3.4×109 A/m2 at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe2As2 superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual δl and δTc pinning models, which suggest the intrinsic nature of pinning in the compound.