I've had this happen to me a couple of times where my iphone would ask me if I wanted to trust a PC that I have told it before that I trusted it & have synced it multiple times before. I didn't understand why it would do this. I don't know why iphone would have a trust issue
What I've found that sometimes when I have iphone plugged into the computer & then turn on my PC, it may ask me if I want to trust this PC. ...But if I wait until the PC is fully booted up, say at the log on screen or after you're logged in, that it won't ask me about a trust issue.
Nowadays, I generally plug it in after I've logged in, so I don't get these prompts anymore.