Check what is the price control maintained for the Material master in Supplying plant and see whether any price is there with respective to Price control or its Zero.
Also share your accounting document screen shot with Transaction key. It could be AUM key.
Generally if you donot maintain the price in Material master for your plant with respect to Price control then AUM key comes into Picture.
Say you have a Material code ABC and you are transferring it from 1100 to 1200 Plant where as Price control for the material in suppling plant is V with a valid price and in receiving plant it is controlled by Standard price "S" with zero price then you get the AUM key instead of stock account BSX.
How ever, we cannot say exactly what was the reason in your case.