bind 9.5.0P1, lots of request but less query , under attack?

[moneyhere at gmail.com]

Hello Everyone
 Our two dns servers run solaris 10 and play Maser and Slave role of
our department.
our bind version are 9.5.0.
recently this two boxes refuse to do recursive response at night 22:00
and keep on for several hours (our users' number will reach the peak
at that duration) . so I collect the "rndc stats" data and make a
gnuplot graphic pictures. Found that:
When that time reached,

1, Request and Query recursion number is as high as impossible
2, Query and Query Success number is very low

What happened to our server ? are they under attack?
for someone wander I upload the pictures to follow urls,

picture when Problem happen
http://www.flickr.com/photos/ryanqian/2943392841/sizes/o/

picture when it runs normal
http://www.flickr.com/photos/ryanqian/2943392843/sizes/o/