Gigantic load average spikes

[rihad]

Postgres 10.3 On freebsd 10.3 is almost idle, disk i/o about 5-10%, 
number running processes about 1-3, cpu about 90% idle, then we run a 
i/o heavy job like "refresh materialized view", cpu & disk i/o still not 
maxed out, all of a sudden the number of running processes increases to 
about 250-300 for a second, which increases load averages that we 
periodically poll to see if everything's alright. Why is that? Disk i/o 
is far from being maxed out, the actual number of running processes is 
really small.