NetworkManager doesn't seem to emitted the StateChanged NM_STATE_DISCONNECTING signal when the system is put to suspend. Instead it just sends the NM_STATE_ASLEEP signal after disconnecting all network devices.
This causes troubles for programs like Pidgin, because they depend on the NM_STATE_DISCONNECTING signal to cleanly disconnect themselves from remote servers.
Also dispatcher scripts in pre-down.d aren't run when suspending the computer.

I am sure if that is a bug, but I think NetworkManager should emit the NM_STATE_DISCONNECTING signal to give programs and the dispatcher a chance to react on the changing network situation.
  
--- how to reproduce ----
# Debian 8.0 Jessie
$ NetworkManager -V
0.9.10.0

# Normal disconnect
$  nmcli device disconnect wlan0
## Listen for StateChanged Signals
$ dbus-monitor --system 'type='signal',sender=org.freedesktop.NetworkManager,path=/org/freedesktop/NetworkManager, member=StateChanged'

signal sender=:1.5 -> dest=(null destination) serial=1335 path=/org/freedesktop/NetworkManager; interface=org.freedesktop.NetworkManager; member=StateChanged
   uint32 30
signal sender=:1.5 -> dest=(null destination) serial=1368 path=/org/freedesktop/NetworkManager; interface=org.freedesktop.NetworkManager; member=StateChanged
   uint32 20
# NM_STATE_DISCONNECTING, NM_STATE_DISCONNECTED are emitted

# Suspend
$ systemctl suspend
$ dbus-monitor --system 'type='signal',sender=org.freedesktop.NetworkManager,path=/org/freedesktop/NetworkManager, member=StateChanged'

signal sender=:1.5 -> dest=(null destination) serial=1573 path=/org/freedesktop/NetworkManager; interface=org.freedesktop.NetworkManager; member=StateChanged
   uint32 10
# only NM_STATE_ASLEEP emitted


References:
Pidgin doesn't disconnect on suspend Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=765892