After a SharePoint 2010 to SharePoint 2013 migration our users complained, that in the multiple Active Directory domain environment they have (I wrote about it recently) the People Picker does not resolve the users the same way it did earlier. Only a subset of the users was resolved, users from a few domains were not included in the results at all.
The reason of this issue is a change in the GetTrustedDomains method of the Microsoft.SharePoint.Utilities.SPUserUtility class. Now (in SP 2013) it includes an extra condition, checking the value of SPWebService.ContentService.PeoplePickerSearchInMultipleForests.
If you need the same behavior as in the SP 2010 version, you should set the value of the PeoplePickerSearchInMultipleForests property to true.
You can achieve it using PowerShell:
$cs = [Microsoft.SharePoint.Administration.SPWebService]::ContentService
$cs.PeoplePickerSearchInMultipleForests = $true
or via C#:
SPWebService.ContentService.PeoplePickerSearchInMultipleForests = true;