I can reproduce your problem if there are no files in the directory that were modified in the last hour. In that case, find . -mmin -60 returns nothing. The command find . -mmin -60 |xargs ls -l, however, returns every file in the directory which is consistent with what happens when ls -l is run without an argument.
To make sure that ls -l is only run when a file is found, try:
find . -mmin -60 -type f -exec ls -l {} +