Weird sponstaneous reboot

I've just experienced a strange spontaneous reboot. I noticed my Nexus 5 was indicating 'updating application n of nn' exactly like what happens with an update. I thought - looks like it's decided to OTA to LMY48M. But no, after 'updating' all the apps it's still on LMY48I. What's going on? I can understand a spontaneous reboot but not one which goes through the process of updating the apps. :confused:

Trev