I can explain in general what causes spikes. (just for some base knowledge)
SNMP ifInOctets and ifOutOctets are counters, which means they start at 0 (at device boot) and count up from there. LibreNMS records the value every 5 minutes and uses the difference between the previous value and the current value to calculate rate. (Also, this value resets to 0 when it hits the max value)
Now, when the value is not recorded for awhile RRD (our time series storage) does not record a 0, it records the last value, otherwise, there would be even worse problems. Then finally we get the current ifIn/OutOctets value and record that. Now, it appears as though all of the traffic since it stopped getting values have occurred in the last 5 minute interval.
So whenever you see spikes like this, it means we have not received data from the device for several polling intervals. The cause can vary quite a bit: bad snmp implementations, intermittant network connectivity, broken poller, and more.
Anyway, just posting this here for posterity, good luck.