Why won't excel read my csv file created with the powersell out-file automatically.

Question

I wrote a PowerShell script that creates a CSV formatted file.  The script simply creates a comma delimited string for each entry and adds it to a collection.  Then the out-file command write it to a file.
When you open it with Excel each line is put into one cell.  If you import the file and specify the "," as the delimiter, it imports just fine.  If the data is saved out again as a csv file,  the file is about half the size and opens
just fine with excel.
If you open the original file and the file created by Excel with notepad, they look the same.
So the files are different in size, contents look the same, but Excel won't automatically open the original file.
Any ideas of why this happens?  Also PowerShell and Excel are both running on Server 2012 R2.  Excel is a remote app.
If I do an export-CSV, I get some kind of information about the object.
Here is the script:
foreach ($group in $groups)
   $header += '"' + $group.name + '",'
   $CSVdata = @()
   $CSVdata += $header
   # Create a user entry
   $users = Get-ADUser -filter * |select SamAccountname, Name | sort SamAccountName
   foreach ($user in $users)
      $Groupmembership = Get-ADPrincipalGroupMembership -Identity $user.SamAccountName
      $userentry = '"' + $user.SamAccountName + '",'
      $user
      foreach ($group in $groups)
          if ($Groupmembership.SamAccountName -contains $group.SamAccountName) { $userentry += '"X",'}
          else { $userentry += '"",' }
       $CSVdata += $userentry
      $CSVdata
      Out-File -inputobject $CSVdata -FilePath c:\batch\GroupMembership.csv

Answer 1

Ok the script works exactly like I want it.  Thank you very much.
I am trying to understand the script but I am unable to figure out what the line "$keys=$t.Keys|%{$_}"
does.  I figured ".keys" was a method but my search for it comes up blank.  Do you have a reference you can point me to?