Text message threads disappeared before my eyes

I've been using a DTEK 60 for about a fortnight now, having very reluctantly moved from a Z10. This morning I opened my messages app, and, within a second or two, had the rather unsettling experience of seeing the two most recent thread headings, or whatever they're called (icon, name of sender, first line of most recent message) disappear before my very eyes of their own accord, leaving two that were more than four days old. I can't find any setting on the phone that might have caused this - can anybody help point me in the right direction? Thanks. Gareth