Do you have your cell setup as a "phone" in Google Voice, which allows you to ring your cell simultaneously? If so, and then you forward your cell to Google Voice via your cell carrier, a call to your cell directly forwards to Google Voice, which forwards it to your cell, which sees you calling yourself and puts you into Voicemail...
Or, it's because you're using GV as your voicemail, so when your cell is forwarded to Google Voice, it sees a call coming in from the cell and figures you are trying to check voicemail...
An easy fix might be to forward the call, if you can, by removing callerid from the call, i.e. blocking the number, which will then work like anyone else calling.