The Mishna teaches that one should not begin a wedding meal or conduct Din prior to Mincha, lest he come to perhaps miss the Tefillah.  We learn elsewhere (Shabbos 10a) that conducting a Din is a Mitzvah, as certainly is attending a wedding.  Why should one not begin the Mitzvah at hand, even if it may lead to forfeiting a later Mitzvah – since we know that one who is involved in a Mitzvah is exempt from doing another?