Most probably, it would be a small mistake in the PHP code of the forum software. But I am not really sure about it.
The following is a similar situation where it would output an extra "," at the end:
Here when joining the array, if the last element in the array is empty or contains only white spaces, then the joined text would contain an extra space at the end.
$users_viewing_thread = "akhileshbc";
$users_viewing_thread = "gep13";
$users_viewing_thread = "guest1";
$users_viewing_thread = ""; //an empty element - this would affect the implode(aka array join) function's output
$output = implode( ', ', $users_viewing_thread );
echo $output; // will output the following string: "akhileshbc, gep13, guest1,"
Output of the above code would be:
Just a guess..
akhileshbc, gep13, guest1,