Your users have bookmarked your $webserver, load goes up.
Solution: more servers and a redirection
table webpool persist { 192.0.2.214, 192.0.2.215, 192.0.2.216, 192.0.2.217 } match in on $ext_if to $webserver port $webports rdr-to <webpool> round-robin
Pre-4.7:
rdr on $ext_if from any to $webserver port $webports -> <webpool> \ round-robin sticky-address