Whenever the increase of Iman is mentioned, it is due to an incident of an event and not deeds being committed.
In Aali-Imraan 3:173,
الَّذينَ قالَ لَهُمُ النّاسُ إِنَّ النّاسَ قَد جَمَعوا لَكُم فَاخشَوهُم فَزادَهُم إيمانًا وَقالوا حَسبُنَا اللَّهُ وَنِعمَ الوَكيلُ
Those to whom the people said, ‘indeed the people have gathered against you; so fear them.’ But it only increased them in Iman, and they said, ‘Allah is sufficient for us, and He is an excellent trustee.’
Here, it was a fearful event.
In at-Tawba 9:124,
وَإِذا ما أُنزِلَت سورَةٌ فَمِنهُم مَن يَقولُ أَيُّكُم زادَتهُ هٰذِهِ إيمانًا ۚ فَأَمَّا الَّذينَ آمَنوا فَزادَتهُم إيمانًا وَهُم يَستَبشِرونَ
And whenever a surah is revealed/sent down, there are those who say, “Which of you has this increased Iman?” As for those who believed, it has increased them in Iman, while they are rejoicing
The increase here was due to the surah being sent down/revealed.
إِنَّمَا المُؤمِنونَ الَّذينَ إِذا ذُكِرَ اللَّهُ وَجِلَت قُلوبُهُم وَإِذا تُلِيَت عَلَيهِم آياتُهُ زادَتهُم إيمانًا وَعَلىٰ رَبِّهِم يَتَوَكَّلونَ
The believers are only those whose hearts tremble when Allah is mentioned and when His signs/verses/ayaat are recited to them, it increases their Iman and put their trust in their Lord
The increase here is by the mere mentioning of Allah and the recitation of his verses or signs, i.e. the creation.
In Surah al-Ahzab 33:22,
وَلَمّا رَأَى المُؤمِنونَ الأَحزابَ قالوا هٰذا ما وَعَدَنَا اللَّهُ وَرَسولُهُ وَصَدَقَ اللَّهُ وَرَسولُهُ ۚ وَما زادَهُم إِلّا إيمانًا وَتَسليمًا
And when the believers saw the confederates, they said, “This is what Allah and His Messenger had promised us, and Allah and His Messenger spoke the truth.” And it increased them only in faith and acceptance.
In Surah al-Fath 48:8,
هُوَ الَّذي أَنزَلَ السَّكينَةَ في قُلوبِ المُؤمِنينَ لِيَزدادوا إيمانًا مَعَ إيمانِهِم ۗ وَلِلَّهِ جُنودُ السَّماواتِ وَالأَرضِ ۚ وَكانَ اللَّهُ عَليمًا حَكيمًا
It is He who sent down tranquillity/calmness/security into the hearts of the believers that they would increase in Iman along with their [present] Iman. And to Allah belong the soldiers of the heavens and the earth, and ever is Allah Knowing and Wise.
Allah says the increase of Iman occurs when He sends the sakeenat into the hearts of the believers.