"When Matthew introduces the reference to the appearance of TO SHMEION TOU hUIOU TOU ANQRWPOU EN OURANWi, 'the sign of the Son of Man in heaven,' with TOTE, 'then,' and introduces the following reference to the actual coming of the Son of Man with another TOTE, he makes it impossible to take the sign as either the phenomena in the sky of v 29 or as itself (as an appositional genitive) the coming of the Son of Man mentioned in the last half of the present verse [Mt 24:30] (contra Gundry; Bruner). Matthew thus apparently regards the appearing of the sign of the Son of Man as something independent of both, but if so, it is very difficult to know what he has in mind. It is obviously some further spectacular event that will by its conspicuousness alert the world to what immediately follows, the parousia itself (cf. the question of v 3). Possibly the 'sign' is the setting up of an 'ensign,' which is often mentioned (see, e.g., Isa 18:3; 49:22; Jer 4:21; 1 QM 2:15-4:17) together with a trumpet call (thus Glasson, Schweizer, Hill)."
As you can see, Hagner thinks that it is impossible for the genitival construction in Mt 24:30 to be appositional in view of the way Matthew employs TOTE. After all, the apostle writes:
KAI TOTE FANHSETAI TO SHMEION TOU hUIOU TOU ANQRWPOU EN OURANWi KAI TOTE KOYONTAI PASAI hAI FULAI THS GHS KAI OYONTAI TON hUION TOU ANQRWPOU ERXOMENON EPI TWN NEFELWN TOU OURANOU META DUNAMEWS KAI DOXHS POLLHS.
According to BDAG(PAGE 1012), TOTE in Mt 24:30 introduces that which follows in time. And not only do we have TOTE in 24:30, but KAI TOTE, "and then." At any rate, we are dealing with a portrayal of subsequent events in Mt 24:30. Matthew thus outlines the Son of Man's "appearance" by using TOTE as follows:
(1) Heavenly phenomena occurs (Mt 24:29).
(2) And then (KAI TOTE), the sign of the Son of Man
appears in heaven.
(3) And then (subsequent to this phenomenon) all the
tribes of the earth beat themselves in grief and they
see the Son of Man's coming.
Of course, I'm willing to hear what others have to say about this matter, but Hagner's explanation makes sense to me (which is not to say I think he's correct). I also recommend that you consult BDAG for more on SHMEION. For an interesting and helpful discussion of Mark's eschatological account (Mk 13:24-27), see GRB Murray's Jesus and the Last Days (Peabody: Hendrickson, 1993), pp. 427-432.