wp-cron.php stuck at times

I check often on my server and have to manually kill php processes pointing to WHMCS cron.
They seem to sit their for hours. We host a few thousand domains and run the cron every 5 minutes.

1720 hostking 20 0 348m 5724 3464 R 99.9 0.3 1636:51 php
12251 hostking 20 0 348m 3680 3464 R 99.6 0.2 1607:05 php

Any reason as to why the cron for whmcs would get stuck?

Thanks