"u" versus "a"

I got intrigued by the "u" in the Latin word "insulto" which is considered
to be a derivat from IE *sel, as well as "salire", "saltare".

How is in fact to explain the change of the vowel in the root here? My
Walde-Hoffmann dictionary does not give any explanation and when one take a
look at "salire" there is no mention about "insulto". My little dictionary
gives the "i:nsulto:" but there is no explanation about the change of "a" to
"u". Or do we have to consider the mention that "l" was influenced by "e"
which followed and thus the previous "e" or"a" became an "o" or "u"?
At least that should be the explanation cf. my little dictionary for
explaining the "u" in "-sulto".
The examples given there are Latin "oliva" from Greek "elai(f)a" > olaiva >
oliva or the one of "homo" which originaly was an "hemo", bonus versus bene
or famulos versus familia.

The "change" appears not very clear at all; the explanation here appears a
bit forced to me specialy when we do know about the alternance "e" versus
"o" in IE roots. Can it be in fact we have to deal with reflexes of "o"
grade here where we have "o" or "u" instead of usual "e", "a", "i" which are
supposed to ve reflexes of "e" grade of the root?

Anyway, it appears curious this alternance in IE of e/o with a _more_ later
alternance e/o this time considered just as a phonetic evolution.

Alex