Why do choirs 'drop' in intonation?

The main cause of dropping in intonation is inadequate vocal technique. This may occur in various ways. The most common way is:

• Inadequate breath support, so rising and high notes are generally not pitched high enough.
  

This is also the reason that rising in intonation is much less common: it is simply not possible to have a 'too good' vocal technique.

Some other causes:

• Vowels are sung too 'dull'. This can lead to a pithless and 'dark' sound, which is prone to a dropping intonation.
•  Insufficient 'coloring' of vowels on high notes or passages. This makes it particularly difficult to pitch and to hold high notes.

Other causes, which however can lead to both too high and too low intonation:

• The fact that a choir does not sings well-tempered. In choral works with many modulations and harmonic sequences, this may lead to bad intonation.
•  A poorly developed musical hearing, leaving poor intonation simply unnoticed.