We've had four attacks now over the last month....Iran is clearly trying to impact oil exports.
So What is Iran's motive for attacking these oil tankers?
The US has put sanctions on Iran's oil exports for about a year now and their economy is in big trouble. The sanctions during the Obama administration were phony because Obama granted "exemptions to India, China, and Japan to continue to import Iranian oil so there really weren't sanctions on Iran at all back then.
But now there are real sanctions on Iran. For about a year Iran has't been able to export much oil.
Its not hard to imagine Iran responding to US sanctions on their oil exports by taking steps that will restrict all oil exports through the Straits of Hormuz.

Nothing big. Nothing that will start a war. But a drone attack one week, then a mine attack the next. Just a series of little attacks that over time will tend to restrict oil coming out of the straights of Hormuz.