Any explosion at a border is going to be treated as a terrorist event until otherwise shown. But cars also generally don't just explode, so I would also assume criminal malice of some kind at least.
The word explosion is in the headline. No government agency or anyone who was at the scene is quoted using that word. (Edit: not in the linked article at time of writing, FBI Buffalo Field Office has used that term in statements) I question whether there even was an explosion.
Cars are fueled by a flammable liquid. The gas can catch fire, and cars do catch fire from time to time. Cars don't have pressurized fuel tanks, so will burn rather than explode.
However, there is enough fuel for a car fire to be large enough that a layperson might mistake it for an explosion.
This could be an accident. It could be a terrorist attack. But we can be sure that for-profit news companies have an incentive to use whatever language sounds most scary. If it bleeds it leads, as the saying goes.
To their credit, I am watching ABC News coverage of this right now over their website and they are being very clear that there's not enough information yet. Some people are saying it's suspicious, but they are not fearmongering.
The day before thanksgiving is a day with lots of people traveling. More people driving means accidental car fires at high traffic locations are more likely. Could be suspicious. But I'm certain if anyone credible described it as a terrorist attack, they would have put that in the headline as its much more clickable.
Some of the other folks pointed out the vapors possible in a fuel tank. I'll add that I had a coworker who had been an explosives disposal fella, and he used to get twitchy if the tank in our car dropped under half. He said it was a relatively small but definitely larger-than-our-car bomb ready to go off. I'd trust his reckonings on that one. Plus, there are parts of a car that, even if its just burning, will explode, such as the tires. I had a single tire blow as I walked around a burning car, and I would not have been amiss in describing it as a small bomb going off.
An electrical fire hitting the gas tank will look like an explosion, especially if there's a lot of vapor in the tank, and especially to a layman. Prior to CGI explosions, pyrotechnics on movie and TV would have jugs of gasoline taped around them to get a nice fireball.
Two occupants of a car that exploded after it sped the wrong way into an inspection booth at the Rainbow Bridge were killed in the blast, according to law enforcement sources at the scene.