Round robin upstream stuck

Server NGINX
1

Hi

     I got some but not that much errer log in error.log like :

round robin upstream stuck on 1 tries while connecting to upstream,
client: xxxx , server: xxxx, request: "GET xxxx”

     I tuned log level to critical, it appears every 10 , 20 or 30

mins, I am using upstream module for load balancing with about 10000+
established connection.
Does it mean that my frontend servers are overloaded or the
backend servers/VIP response is too slow?

my “frontend server” directives looks like :
proxy_connect_timeout 10s;
proxy_send_timeout 10s;
proxy_read_timeout 10s;
proxy_next_upstream timeout;

     and my “upstream “ directives looks like:

upstream mini.tudou.com {
xxxx;
backend server xxxx:80 ;
backend server xxxxx:80 ;
}

Thx
Jie