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503 if backend node goes down

I have following configuration,

log local0 #debug
log /dev/log local0
log /dev/log local1 notice
chroot /var/lib/haproxy
stats socket /run/adminha.sock mode 600 level admin
stats timeout 30s
user haproxy
group haproxy

log global
mode http
option httplog
timeout connect 5000
timeout client 2m
timeout server 2m

frontend outwards-haproxy
bind *:80
bind *:443 ssl crt /pathgiven no-sslv3
redirect scheme https code 301 if !{ ssl_fc }
stats enable
stats uri /stats
stats enable
stats uri /stats
stats hide-version
stats auth admin:1234

acl is_off url_beg /smoke1/
use_backend offapp if is_off

backend offapp
balance roundrobin
option redispatch 1
retries 2
cookie platform_extension_sessionid prefix nocache
server off1 host1:8443 check ssl verify none cookie s1
server off2 host2:8443 check ssl verify none cookie s2

I m getting 503 if one of backend server is stopped. Why it is not seamlessly transferring connection ?? even if i have 2 nodes