max-cache-size query

[Techi]

Hallo,
Recently, I faced huge problems with my DNS servers (bind crashed with no 
apparent reason). Some of the symptons were:
* Huge number of connections on our firewalls (>150000).
* A lot of errors in syslog about max file descriptors limits reached 
(currently on system, the FD limit is 4096, the default of centos)

Anyway, after the proposal of a friend of mine, I removed the the max-cache-
size limit (that was set to 256MB.
After a restart of bind, the FW guys reported a huge drop on connections 
(<10000)!
Additionally, I have no crashes so far (in contract with 1-2 per week).
So, why:
a. bind generated so much traffic?
b. Is it possible to have bind crash because I could not handle the cache 
clean-up and on the same time to serve requests?

Thank you