Hello,
By default, your multiple hub servers can load balance automatically in your organization.
Per my known, I consider this behavior is normally.
Because there is a healthy server selector which detect these servers that are unavailable in exchange 2010. And when the server is marked as unhealthy, the healthy server selector will ensure that connections to that specific server are tried again to detect
when that server comes online. But there is retrycount.
Please refer to the "Healthy Server Selector" subject in the following article.
Understanding SMTP Failover and Load Balancing in Transport
http://technet.microsoft.com/en-us/library/ff634392(v=exchg.141).aspx
Cara Chen
TechNet Community Support
