Rebbe,
1) The Gemara says that the kohanim interrupted the avodah to go say kerias shema. Why? Haosek b'mitzvah patur min hamitzvah. Even if people were burying the dead those directly involved wouldn't have to say kerias shema. A choson on his wedding night is patur from kerias shema etc.
2) I had the impression that it was actually too early for kerias shema when they left to recite it. Is that so?
B'kavod
Sam Kosofsky
Dear Sam,
Thanks for your excellent questions.
1) The Tif'eres Yisrael (Tamid 5:1) asks this question. He explains that indeed the Kohanim were not obligated to recite Shema. However, they wished to daven before offering the Tamid so that it should be accepted l'Ratzon. For this reason they recited Shema beforehand so that they can precede davening with Divrei Torah.
2) The Gemara (Yoma 37b) states that one who recites Shema with the Anshei Mishmar is not Yotzei since they recite it too early. The Tif'eres Yisrael (ibid) explains this to mean that they did not read it at the preferred time (just before sunrise), not that they actually read it too early.
Kol tuv.
Y. Landy