Some nutrient deficiencies will cause "meristem" (growing tip)
necrosis, boron if I recall correctly
(
http://4e.plantphys.net/article.php?ch=5&id=289 ).
I imagine that there are different kinds of diseases that can
cause it, too, though, as well as competitive oversupply of
some other nutrient that is blocking uptake of what the plant
feels is lacking.
Jeff Fleming's description of Cherokee Purple in Tania's
TOMATObase says that Cherokee Purple does not keep
producing indefinitely for him, that after some number
of fruit it just stops (whether they have had a frost or
not, regardless of the fall weather, etc). So maybe it is
just the cultivar (a rare variation on determinacy in tomatoes,
where it grows and fruits like an indeterminate all summer and
then when it is done, it is done, end of story).