These ULLs are called TUTT lows...

A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an "upper cold low". TUTTs are different than mid-latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995). Courtesy of AMOL (NOAA)."

Bonnie has been pestered by a TUTT low since birth, currently in the Western GOM. It and the upper ridge over the SE USA has been the system's steering mechanism since day one and Wind Shear analysis has made that point very clear. Frankly, it was a fluke it formed into a depression, but there was one day where the shear lightened considerably before making its presence known the following day. Bonnie's undoing was her speed. The thinking originally would be for the ULL to get out of the way; however Bonnie was moving so fast that it never got away from the ULL winds aloft. Bonnie never had an opportunity to gain any vertical depth thus was steered by the tropical easterlies that are evident this time of year.