What is the reason behind band gap narrowing in semiconductors

I want to know why some semiconductors band gap decreases after doping with elements. Burstein-Moss band-filling effect can be useful to explain band gap widing in a semiconductor materials but i was unable to find any logical explanation for band narrowing effect. Can you please explain the mechanism of band gap narrowing.

This below quotes were taken from a research article.

There is general agreement that two competing phenomena are dominant in affecting the absorption edge in heavily doped semiconductors. First, the well-known Burstein-Moss band-filling effect which shifts positively the measured band-edge energy with increasing carrier concentration. In this case the measured optical gap $E_{m}$ is the sum of the optical gap of the lightly doped material $E_{0}$, plus that due to filling of the conduction band due to Is $\Delta E_{BM}$, I. E. $E_{m}=E_{0}+E_{BM}$. Thc second phenomenon which affects the optical absorption edge with increasing donor density is due to a change in the nature and strength of the interaction potentials between donors and the host crystal. This latter effect gives rise to a band-gap shrinkage and to some increased tailing of the absorption edge. In this case the measured optical gap is $E_{m}=E_{0}+E_{BM}-\Delta E_{g}$., where $\Delta E_{g}$ is the gap shrinkage.

I didnt understand the explanation given in second phenomenon. What is author meant by due to a change in the nature and strength of the interaction potentials between donors and the host crystal.