We run a CARP setup with two dedicated Xeon Servers and pfSense 2.5.1.
For HA there's a dedicated 1G copper link. Everything else is connected over 2 times 10G LACP. For every downstream interface there's a VLAN interface on top of that LAGG.
We're monitoring both nodes with Icinga2 and NRPE individually. We recently switched from Nagios to Icinga2 and also from IPv4 addresses to IPv6 addresses. The problem started occuring after this switch.

Only the secondary device suffers from the problem that the NRPE3 process itself after a couple of minutes takes 100% CPU load. After some more time even more NRPE processes spawn, also with 100% CPU load. After few hours load goes higher than 5. Never waited much longer but I guess it's a neverending story.

NRPE process looks like this:

nagios  36022 100.0  0.0  16144  6192  -  R    13:40       7:27.85 /usr/local/sbin/nrpe3 -d -c /usr/local/etc/nrpe.cfg

config file looks like this:

log_facility=daemon
pid_file=/var/run/nrpe3.pid
server_port=5666
nrpe_user=nagios
nrpe_group=nagios
allowed_hosts=2a00:1234:0:106::61
dont_blame_nrpe=0
debug=0
command_timeout=60
connection_timeout=300
command[check_users]=/usr/local/libexec/nagios/check_users -w 5 -c 10 
command[check_load]=/usr/local/libexec/nagios/check_load -w 15,10,5 -c 30,25,20 
command[check_root]=/usr/local/libexec/nagios/check_disk -w 20% -c 10% -p /
command[check_var]=/usr/local/libexec/nagios/check_disk -w 20% -c 10% -p /var/run
command[check_zombie_procs]=/usr/local/libexec/nagios/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/local/libexec/nagios/check_procs -w 150 -c 200 
command[check_swap]=/usr/local/libexec/nagios/check_swap -w 50% -c 25% 
command[check_synclink]=/usr/local/libexec/nagios/check_ping -w 10,2% -c 20,5% -H 192.168.15.2
server_address=2a00:1234:0:16::3

It looks like sometimes the icinga2 daemon gets a timeout when connecting to the nrpe process and it somehow looks like at that event a new nrpe3 process is being spawned.
I wasn't able to find anything related to that.