This appears because the server you are accessing is configured to prevent too many accesses within certain time periods. i.e. no more than 5 hits in 10 seconds and no more than 30 hits in 300 seconds.
If the IPS is "simple" it will be quite efficient at generating false positives and blacklisting legitimate users. The most common occurrences are when multiple users accessing the site from one IP address, through home connection with NAT.
Extract from a Fortinet router config file :
config system replacemsg nac-quar "nac-quar-ips"
set buffer "<html><head><title>Attack Detected</title></head><body><font size=2><table width=\"100%\"><tr><td bgcolor=#3300cc align=\"center\" colspan=2><font color=#ffffff><b>Blocked because of IPS attack</b></font></td></tr></table><br><br>An attack was detected, originating from your system. Please contact the system administrator.<br><br><hr></font></body></html>"
set header http
set format html
end
Most times such messages originate from false positives. Sometimes the addition of AJAX calls on a website increase such false positives due to increased page requests.