"Why is that?" 

  After describing how the hamsah "geese,"
krauncah "curlews,   and mayurah  "peacocks"
chant the Buddhadharma in Sukhavatt, Sakyamuni
Buddha asked his disciple Sariputra if
he thought the birds were tiryagyonigatah,
born as animals because of karmic retribution.
Before Sariputra could answer, the
Buddha said: Na punar evam drastavyam, "It
shouldn't be seen that way." Now the Buddha
asks:  tatkasmaddhetoh,  "Why is that?"

  Tat is the nominative neuter singular of
the demonstrative: pronoun "this/that,"  In
the Sanskrit writing System, the final -t
is joined to the initial k- of kasmad, but
they are separate words.

   Kasmat is the ablative singular masculine
of the interrogative pronoun "who/what?"
Here it is written kasmad, final unvoiced
-t changing to voiced -d before the initial
h-  of  hetoh. The initial h- of hetoh  is
in turn influenced to became dh-, and for
economy in writing, the two words are also
joined in the script.

  Hetoh is the ablative singular of the
masculine noun  hetu "cause/reason."  The
ablative case here expresses cause or origin,
and so kasmad-d-hetoh means "for what
reason?" and tad kasmad-d-hetoh literally
asks, "This for what reason?" That is, why
shouldn't we consider the birds in the Land
of Happiness beings that have fallen into
the destiny of animals, one of the three
evil destinies?