Contacts -> Account Links Disappearing

Contacts -> Account Links Disappearing

I had all of my Contacts assigned to Accounts. When I logged back in the next day, I noticed many of my contacts had no account assigned to them. So I went through each one and made sure they were all assigned and linked to a valid Account.

Today I saw the same thing - about 1/3 of them had no assigned account! So for some reason the link is being broken, or not holding.

Has anyone else experienced this issue?  What can be done to prevent this from happening?