Why do Christians stand when we sing? In my experience, every church I've either been in or seen recorded ask the congregation to stand when they sing. Without having done a scientific study, it seems this is a near-universal practice among Christians (and please correct me if this is wrong).
Is there a biblical basis for this, or is it merely a wide-reaching convention? If so, is there anything to suggest when or how this practice may have begun?
I recognise this question may be at the border of what's on-topic. I've tried to keep in on-topic by not asking why we sing, but for the biblical basis and history of this specific Christian practice. Given this practice seems to be the same amongst many denominations, I hope it's ok not to specify a particular group.
Why do Christians stand to sing ?
I think the reasons are fundamental to the act, so I do not see biblical instructions or precedents to be relevant.
One may ask, how one should eat a meal : should one sit, or recline ? One may ask, how should one read the bible  : should one be seated or kneeling ?
These are matters of preference and matters of practicality, I suggest.
So I am answering under your suggestion of 'wide-reaching convention'.
After a comment mentioned common practice in Germany, I have to maybe say that my own experience is solely in the UK and solely within Protestant Trinitarianism.


Often, God is being addressed in what is being sung. So : reverence is one reason. This is expressed both individually and corporately, by an entire congregation, by standing.

For proper singing, the lungs need to be filled to capacity, not to make a loud noise, but in order to sustain a note and to sing the whole line without pause.


Abdominally, standing is a better posture than sitting, if one wants to repeatedly fill the lungs and exercise the abdominal muscles.

Singing is a physical activity, sometimes accompanied by (small) body movements or movement of limbs in an expressive manner, so standing is better than sitting.

I don't mean anything excessive by these movements, I refer only to very slight swaying in time to the music, or the tapping of a foot to keep rhythym and to very small gestures of expression as the heart engages with the words and with the melody.
Singing for spiritual purposes is an engagement of the whole person : spirit, soul and body. It is active. It is focused.
And it is very effective.

Paul encourages singing of psalms and hymns and songs in his epistles and the psalms repeatedly encourage singing and praising throughout that book of one hundred and fifty examples of praise, worship, petitioning, pleading, lamenting, crying out and triumphing.
I don't see there is any commandment or statute regarding standing to sing in the bible but there are examples which are worthy of note. Just as nowhere in the bible am I told to pray three times a day, yet I read that Daniel did so : not that he advertised the fact but when people put him under surveillance, they couldn't help but notice.
For example, there is a hint in Psalm 134 [KJV version]:

(1) Behold, bless ye the Lord, all servants of the Lord. which by night stand in the house of the Lord.


(2) Lift up your hands in the sanctuary and bless the Lord.