History of the English language
What causes slips of the tongue, includingmetathetical slips of the tongue, in the first place,
and why some people are more prone to them than others is not yet clear: more needs to be
known about how the brain works. However, for the present purposes this question is
immaterial
Why do metathetical forms oust old forms?
ease of pronunciation (suhkrut-suhkurt)
analogy-Nucular pro nuclear,Cf circular, muscular
Proto-Indo-European *kailo-"whole, uninjured, of good omen"
Proto-Germanic *hailaz
1. Old English hal HALE (sound in health, vigorous, robust (HALE AND HEARTY),
WHOLE
2. Old English halsum WHOLESOME (e.g. WHOLESOME FOOD)
3. Old Norse heill (healthy) HAIL (as a greeting), TO HAIL (to greet, also: to hail a
taxi, also fig. to praise highly, to acclaim, as in "critics hailed her new book"),
WASSAIL; German "Heil!" not used any more ("Heil Hitler! and the associated shame
(just as with Reich)
Germanic *hailitho > Old English hælth HEALTH
annaabi.ee 
