Which JGroups release do you have ? I hope a more recent one...
You mentioned the 4 appservers are connected to 3 switches ? Did you mean that each instance has 3 NICs, each connecting to a different switch ?
If that's the case, make sure you don't have multicast loops in your network, where switches forward IP multicast traffic to each other until the TTL is decremented to 0.
Can you reproduce this ? If so, can you connect all 4 hosts directly to 1 switch only, to see if this happens again ?