HAWC measured the angular profile of the TeV $\gamma$-ray halo around Geminga, which indicates an anomalously slow diffusion for the cosmic electrons and positrons (electrons hereafter) in this halo region. The origin of the slow-diffusion region is a fresh and intriguing problem. We first check the self-confined scenario, that is, the electrons released by Geminga induce Alfven waves through streaming instability, and they are then trapped by the Alfven waves. Considering the proper motion of Geminga pulsar, however, we find that Geminga cannot provide enough electrons in the late age to account for the so slow diffusion. Then we propose another scenario in which the slow-diffusion region is preexisting. Geminga may still be inside the turbulent region left by its parent SNR. The SNR injects energy to the magnetic field turbulence of the background medium, which can be adequate to explain the observed slow diffusion. Some other TeV halos could also be understood under this scenario.