Since long a sect of people have been propagating a campaign of Not to stand up during the Boi ceremony, as the Priest recites ‘Dushmata-Duzhukta-Duzvareshta’ to drive out Ahriman and that standing up would be an Honour to Ahriman and an Insult to The Atash Padshah .
The other believes that we are removing the evils from the world & so the Bell ringing defeats Ahriman.

Shri Rohin Noshirwan Karanjia of Surat has taken this matter up in his Gujarati article in the Souvenir of the Navsari Atashbehram celebrations October 2015. Some brief points from it:-

All religious ceremonies are done by the Ervard sahebs in a demarked area ‘A Paavi’. Religious Books states, “Any Religious ceremonies done within the Paavi the pure energies emanating while reciting Avesta Maanthras can also reach out of the Paavi region, but the foul energies of Ahriman CAN NOT enter the Paavi region. Thus Paavi acts as a protective cover & the spiritual strength emanating from it destroys outside enemy attacks. So the purity of the ceremonies done in a Paavi is not defeated”
It is true that the words ‘Dushmat-Duzukt-Duzvarsht’ do mean ‘Bad Thoughts-Bad Words-Bad Acts’ but taking grammar rules into consideration the words appended with them, ‘Oem Goft- Oem Kard-Oem Jast’ come along, which without these words the Dushmat… words remain incomplete. As per Erv. Edulji Kanga the words portray, “In my Past Life – This Life – Or the Life in Future; if I have had Bad Thoughts-Words-Or Acts; or in future If I may have them; then I am distraught, I do penance for it, I ask Forgiveness and promise to be away from such conduct in future.” So as the Ervard saheb rings the Bell during the Boi ceremony uttering these words, there is no negative vibes felt. Only positive energy is spread. Hence one should stand up during that time. Remember the Ervard saheb does the Full Boi ceremony while remaining standing.
The Kadmi Boi ceremony is a bit different from the Shehenshai one. They utter, “Saoche Buye Ahmay Namaane” (3 bells rung) “Mat Saoche Buye Ahmay Namaane” (3 bells rung) “Raochahi Buye Ahmay Namaane” (3 bells rung). Meaning, ‘O fire may you keep ever burning brilliantly in this house’. At that time there is no issue of Ahriman being present, as there are no words uttered that seem to represent Ahriman coming to douse the Atash, and so when we stand up to honour the ringing of bells, we do not honour Ahriman.
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Leaving that entirely aside, let us take it up from a Non religious angle. Simple straight forward earthly reason.
We call our consecrated Atash as ‘Atash Padshah’- The King! In olden days when there was Kingship in the world, there would be a fixed time when the King would arrive in his court Room. To Mark the occasion & warn all, there used to be a Ringing of Bells to alert the genera that the King was coming to his court room. (In India There was also an announcement in the court yard before the King entered) When the King entered everyone would stand up as a mark of respect and alliance to the king.
Also to Mark the Change of Time they used to ring the bell to let the common people know of the Time. (Remember the ringing of Bells in Schools to mark the end & start of a Period or Recess)
As we consider Atash as son of Ahuramazda, (Atash Puthre Ahuremazdao) and a Padshah – (A King) When The Padshah is offered a Machi & the Boi (Actually from Gujarati KHUSHBOI -‘Fragrance’) The Fragrant offering to the Atash at the time of the change of the time -‘Gah’ from Morning to Noon & Evening to Night etc. So as a Mark of respect to The Atash Padshah & the Bringing in of the New Gah, we rise up when the bell rings proclaiming the ‘Kings’ entry.
And hence we must stand Up when the Ringing of the Bells at the time of offering Machi.