Degassing HI from Reaction (Via Ultrasound Maybe?)
In short, to answer your question &quot;Why side products are being produced&quot;,  you appear to have ignored all possible radical activity!
Per this source: Free radical formation from sonolysis of water in the presence of different gases, for example, to quote:

$\ce{H2O + Sonolysis -&gt; .H + .OH}$

Here, replacing H2O with HI likely results similarly in:
$\ce{HI + Sonolysis -&gt; .H + .I}$
Some possible further reactions:
$\ce{.H + .H -&gt; H2 (g)}$
$\ce{.I + .I -&gt; I2 (s)}$
And, in the presence of an organic (RH):
$\ce{.H + RH -&gt; .R + H2 (g)}$
$\ce{.OH + RH -&gt; .R + H2O (l)}$
$\ce{.R + .I -&gt; RI }$
$\ce{.R + .R -&gt; R2 (g)}$
Also, with any dissolved oxygen (or even nitrogen, see cited reference), expect even more, as for example:
$\ce{.H + O2 -&gt; .OOH}$
$\ce{.HO2 + .HO2 -&gt; H2O2 + O2}$
$\ce{.R + .OOH -&gt; ROOH}$
....
As such, expect a range of products, as would occur, by the way, if you performed photolysis in lieu of sonolysis. However, there are related reactions, like in the presence of ammonia, where the final photolysis products can display a high degree of product selectivity.