B C D F G H J K L M N P S T V Z
These are the suspects!
When a passage has the consonants removed, the singer usually finds his way from note to note more easily. Practicing songs as vowel-phrases can be very helpful for finding the musical flow.
But we are blessed and cursed, but mostly blessed, with real words to sing. The consonants interrupt the flow. They must happen quickly and lightly. The problem is, when singing strongly especially, there is a lot of acoustic and breath energy leading up to the consonant which can make the consonant pressurized enough to mess up efficient phonation and require a recovery to get back on track.
As a general rule, the louder you sing, the softer you need to make the consonants in relation to the sonorous vowels. Listen to how Italian and Spanish speakers pronounce D and T – much less force than in English. It can be instructive to play with these foreign sounds. If you are singing high in your range fortissimo and you spend too much time and energy pronouncing the consonants, you can get into trouble. Maybe not on the first phrase, but over the course of a song or set, overpronunciation can add up to fatigue and more likelihood of cracking.
When a phrase is repeated in a song, the repeats can often be sung with looser diction, with the music overtaking the words. This goes hand in hand with repeated phrases often being at higher or louder pitches than the original phrase. I wrote about “letting go of the vowel” already. Letting go of sharply defined consonants also has its place. Relativity and all that, as it relates to an expressive performance.