One valid argument I have is that all Newar castes have specific occupational roles to perform in soceity (like Jyapu = farming, Bare = goldsmiths, Kasai = butchers, Sayami = oilworkers, Rajopadhyay = Hindu Brahmin priest, Bajracharya = Buddhist priest, Chitrakar = painter, Prajapati = potter makers, Deula = cleaners, Kapali = tailors, Nakami = ironsmith, Tuladhar, Tamrakar, Kansakar = merchants, etc.) Likewise, Shrestha's traditional role in society had always been in governing and administration since the Malla times. They are the Chhetris of Newars, there is also a Newari word 'Chathariya' / Kshatriya (?) to denote to few upper-caste clans among the Shresthas (like Malla = kings, Pradhan = chief ministers, Rajbhandari = royal treasurers, Raya = advisers, Kayasta = writers, etc.) who are the Kshatriya caste descendants of Malla and Lichchavi dynasties. 

And since they were the controllers of their society (just like how Bahun-Chhetri are of the Khas society, or how Terai Brahmin-Rajput are of Madhesh), they too gained substantially and over-time had over-representation in all fields.