It's easier to understand if we talk about machines we're all well familiar with.

Take a washing machine as an example. It has some settings of different speeds to spin at, as well as some fail-safes built in which is supposed to stop it from spinning if it detects some kind of problem. Even when operating according to its standard parameters, you may find some washing machines will shake violently while spinning.

Now if such a machine were hacked to remove all its fail-safes, and told to spin at even faster speeds (assuming the motor is capable of doing so), there's a good chance for something to go very wrong here. The machine may start jumping off the floor, parts may begin flying off of it, and perhaps the motor might even heat up and ignite.

As another example, think of an oven. Typically ovens have some built in mechanism to maintain certain temperatures, such as exhaust or periodically pausing its heating process. What if it goes out of control and just continually gets hotter and hotter? At some point it's bound to ignite something.

In this case, we're talking about a gas system. I don't know all the details regarding what exactly it was doing. However it probably included a motorized system to move the gas and maintain pressure. If that system malfunctioned in a way where it would ignite the fuel, a huge explosion would ensue. There's many ways for such a thing to go wrong. Such as motors burning up, or too much pressure somewhere causing a rupture and the gas meets damaged electric cables.

Lots of fail-sages are built into all kinds of machines to prevent them from exploding. Sometimes you hear about devices exploding due to defects, such as the batteries in phones and laptops. But sometimes it's hackers. They can modify electronic systems to remove their fail-safes and cause operating conditions that are outside the expected parameters for safe operation.

Iran's nuclear program was destroyed by a virus in their system which caused their uranium enriching centrifuges to spin out of control, and ended up blowing up their facilities.

With all the cases recently in the US with farms, processing plants, and pipelines either totally malfunctioning, or exploding, but no visible suspects, it seems like there's hackers at work here. They remotely access some systems that are foolishly connected to the internet (for remote monitoring and updates of course), and are using some exploit to cause them to not operate correctly. In some cases this causes mild damage, and in other cases, the facility is burned down. We're under attack, and no one is openly acknowledging it.