Below are links to passages where possible causes of addition are discussed:
A-ACCID (addition due to transcriptional or other accident):
Matt 4:23 περιηγεν ολην την Γαλιλαιαν ο Ιησους
Matt 6:1 προσεχετε
Matt 7:29 γραμματεις
A-COMMON (addition in favor of a more common form or expression):
Matt 4:4 ανθρωπος
A-HARM (addition due to harmonization or attraction to the usage in a similar, nearby, parallel, or OT passage):
Matt 3:6 Ιορδανη
Matt 3:11 αγιω
Matt 4:4 ανθρωπος
Matt 7:10 και
Matt 7:21 ουρανοις
Matt 7:29 γραμματεις
A-ORTHODOX (addition in favor of a more orthodox or theological preference):
Matt 4:4 ανθρωπος
A-STYLE (addition to improve style, grammar, syntax, including toward a more classical expression):
Matt 5:39 την δεξιαν σιαγονα
Matt 6:1 προσεχετε
Matt 7:10 και
A-VERSION (addition due to versional, patristic, or lectionary influence):
Matt 3:6 Ιορδανη
Matt 3:11 αγιω
Matt 7:29 γραμματεις