I have no style that uses small caps, but just pasted copy which is conventional upper/lower, and InDesign displayed it as small caps.
what could cause this?
I have no style that uses small caps…
It doesn't necessarily have to be a style. It could be a local override.
Could be an open type alternative.
Highlight the text
Character Panel (Window>Type & Tables>Character)
Sub menu top right
Open Type drop down list
Anything in [ ] is unavailable as an open type feature.
Anything not within the square brackets is available.
If there's a tick mark beside something then switch it off.
Make sure that small caps is not already selected in the Control Panel (top of screen under File, Edit etc.)
Select the Text tool.
The Control Panel will change to the Text options, ensure that the small caps icon is not active.