Officially this is about water rights, unofficially some sources say that it is a coincidence that the US left some much equipment in there when they retired in a hurry from Afghanistan. That equipment is now being used against Iran. Do you believe in coincidences of this kind? Apparently they have a mixture of super-old soviet era captured stuff and more or less recent light armour from US.
Before the US-led force withdrew from Afghanistan it trained and equipped the Afghan armed forces. Most of their armories were US made. When the Taliban overthrew the US-backed government most of these weapons entered into the hands of the Taliban. According to
aljazeera the Taliban forces took control of more than 300,000 light arms, 26,000 heavy weapons, and about 61,000 military vehicles during the takeover of the country. So it is possible that U.S. armored Humvees and Navistar 7000 transport trucks we saw in the video are part of the spoils of war from the withdrawal of the US-led forces and the fall of the Afghan government.
I can't certainly state that the withdrawal of the US-led forces is a calculated attempt to hand over weapons to the Taliban to fight Iran because I don't have evidence to back up my claims.