Multiple motor bike and van travellers have crossed Iran without incident Youtube is full of them.
I suspect they flew a drone or got too near a military facility.
Or could it have been this.... (dumb to say the least)
"Lindsday was conducting a research project during their journey, asking people what constitutes a "good life". She was due to present her findings at a conference in Brisbane in July.
Their final social media updates painted a positive picture of their experiences in Iran. One post showed Lindsay meeting with a religious leader at the Madrasa Naseriyeh in Isfahan."
https://www.gbnews.com/news/british-couple-detained-iran-distressing-situation