This is basically an artifact of using the SGP4 algorithm outside its valid scope. The SGP4 algorithm is a highly simplified approximation that can be used to propagate orbital elements generated by a compatible generator for stable orbits. A decaying orbit can not be accurately predicted using SGP4.

There are some hacks in Gpredict to detect decayed satellites but - as this example shows - they don't work for every case. Introducing an additional check for negative altitudes may solve the problem now, but there will be times in the future when the last available TLE for UARS will give a positive altitude again. The only safe way to solve this is to remove the satellite from the database.