I think I found the root cause. I have snort installed (sorry failed to mention that).
Something changed in the snort.org rules that I usually select.
Last night, snort.org site was down and with a clean new install it failed to update the snort.org rule set but the emerging rules were applied. I kept an eye on the system logs and didn't see the apinger issue. I had even rebooted my 48 port L2 managed switch so I thought it might be the cause (though I had rebooted it earlier).
All through out the night and whole day today there was not a single apinger issue reported in the system log. Just now I did a manual snort rules update and snort.org rule sets were added. I selected the usual snort.org rules (I do the same in the emerging rules too). The moment snort restarted with the new rules applied the system log started to fill up with apinger issues again.
Now I have deselected all snort.org rules and will have to select one rule set at a time to pin point exactly which rule set is causing this apinger issue.