The simple answer may be that the outdoor course you used for the test was too hilly. This was the likely cause of the negative value. Furthermore, I presume that running outside you attempted to keep your HR constant–but due to the hills your HR had quite a large variation. Options would include: re-doing the test on flatter terrain after a good warm-up, or performing the test on treadmill (note that on a treadmill you keep the pace constant, and not necessarily the HR). Fortunately, any Z2 training run can be easily be adapted to be an AeT drift test. So performing it routinely is often easy to do.
Hope this helps,