Jboss lost connections to MongoDB "WARNING [com.mongodb] emptying DBPortPool to /x.x.x.x:27017 b/c of error"

Jboss lost connections to MongoDB "WARNING [com.mongodb] emptying
DBPortPool to /x.x.x.x:27017 b/c of error"

I have a few Jboss 5.1 servers which uses MongoDB data set. Till yesterday
I had 3 Mongo 2.0 nodes in this dataset. Yesterday I added 2 Mongo 2.4.6
secondaries to this dataset; and after that I see many warnings like that:
WARNING [com.mongodb] emptying DBPortPool to /x.x.x.x:27017 b/c of error
"Many" - about 30/day instead of 5/day before. Another thing that I don't
understand - according to jboss log, it started to use 2 new nodes, even I
didn't add them to mongo-ds.xml.
Any ideas why number of dropped connections increased and why jboss
started to use new nodes?
TIA, Vitaly
my mongo-ds is:
<mongo:mongo host="none"
replica-set="mongo1:27017,mongo2:27017,mongo3:27017">