It is the opinion of the Hanafi scholars that when the Quran is being recited one must not speak or perform any actions, but listen attentively.
Therfore to kiss one`s thumbs or say SubhanAllah whilst the quran is being recited would contradict this. Allah has commanded that;
“And when the Quran is being recited listen to it attentively and remain silient, so that you may be shown mercy (7:204)
Imam Ahmed Raza Khan (May Allah shower His mercy upon him) says that there are three times when it is not allowed to kiss the thumbs;
1. When the Quran is being recited
2. When the Friday Khutba is being read the audience must listen to attentively and
3. Whilst the person is in Salaah.
Of course Allah knows best
[Answer provided by: Muhammad Salim Ghisa]