The same thing is happening with the electric car: Some states are now proposing a per/mile car usage tax (where a mandatory black box needs to be installed in your car to track it) to make up for the lost gas tax revenue.
Light vehicles don't do a lot of damage to roads, but in many areas weathering takes quite a toll, so it can make sense to charge all users rather than push it all on shippers or whatever.
Also, do you have an example of governments taxing solar? The trend I'm more aware of is net metering, which is a great deal for the homeowner (the wholesale value of electricity is lower than household metering, on an open market the power company would not purchase power for the retail rate).
There are two purposes of the gas tax: (1) To pay for roads and (2) to provide incentives to conserve a valuable, limited resource (gas).
If we switch over to electric cars, taxing electricity makes sense from the standpoint of (2) because it's the valuable, limited resource that's being consumed by driving.
From the standpoint of (1), literally any source of government revenue will do. You can tax electricity. Or you can tax property (a lot of land's value, especially commercial / industrial, goes away if it isn't connected to a good road system). Or you can just use the general fund (basically everybody benefits from the enormous cross-industry economic gains of having functioning transportation infrastructure, so everybody should pay for it).
Hawaii is putting a stop to solar. Taxing the panels and making it more cost effective to use standard electricity.
It's also the case that Hawaii is a really special case. I wouldn't look to it being the beginning of a trend.
(Of course net metering will probably decline over time elsewhere, but this is not really a tax.)