You have a couple of problems.
First, you can't just add a value to the element you add multiDatesPicker to and expect that date to be selected because it will clear the values. Instead, use an alternate field, like this:
<div class="box">
<input type="text" id="from-input">
</div>
<input type="hidden" id="orig-dates" value="<?php echo $fetch['sel_date']; ?>">
Then in your JS, you first create the datepicker, then add the dates to it:
$('#from-input').multiDatesPicker();
var orig_dates_str = $("#orig-dates").val();
if (orig_dates_str.length > 0) {
var orig_dates_arr = orig_dates_str.split(",");
var dates = new Array();
for (i=0; i<orig_dates_arr.length; i++) {
numbers = orig_dates_arr[i].split("-");
d = new Date(numbers[0], numbers[1] - 1, numbers[2]);
dates.push(d);
}
$('#from-input').multiDatesPicker('addDates', dates);
}
Then your second error is that the format "MM/DD/YYYY" is not recognized. Instead, you need to do "YYYY-MM-DD".
In PHP you can do this like so:
$date = new DateTime($fetch['sel_date']);
echo $date->format('Y-m-d'); // Put this into the <input>
See it on JSFiddle