The reason is that FileZilla uses too many concurrent sessions to
upload files - basically flooding the web server with requests to the
point where the server thinks it is being attacked and then it blocks
you temporarily so that you don't ruin the server for all other users
sharing the server.
If you are using FILEZILLA as your FTP client, do the following:
1. EDIT
2. SETTINGS
3. TRANSFERS
in that window modify the following parameters, to have all the same value:
Maximum Simultaneous Transfers: 3
Limit for concurrent Downloads: 3
Limit for Concurrent Uploads: 3
or modify the value until you found one that is ok for you. In my case 3
is the optimum with my Internet connection and with my server.
Doing this you will stop those blocked errors with your FTP transfers.