Should black high temperature ABS flow in direct sunlight?
Are you sure that this is ABS? (Since it is already trash, you can make a fire test. It should [correct me if i'm wrong] produce black smoke) Some ABS can becomes soft at already  80°C.

Just for fun I measured the temp of my gray sofa that was directly in sun light: It was over 70°C. So it could be possible that the sun light already was enough to weak the black ABS. Also the ground is interesting. Some idi**** room mate puts my black ¿abs? alarm clock on a metal windowsill because he want to know the temps there in the direct sunlight... the alarm clock becomes deformed :(

Probably white and/or coated ABS would be better. Also a inner rod would help against deformation.