In RollerCoaster Tycoon, do vehicle crashes always have a probability of occurring?

Without looking at the specific ride in question, there are a few possible causes:

Guest weight variation

There is some variation in the weight of a guest. It is possible to create a dinghy ride that 'almost crashes' with two average guests, where it will crash if two fat enough peeps happen to share a boat.

You can prevent this type of crash by making sure that sections with strong lateral Gs and negative Gs use covered pipes.

Note: The same underlying cause can cause boats to bump into eachother if you have a slide with relatively high ride duration and/or relatively short minimum time between two dinghies leaving the station. This type of crash usually involves two or more dinghies crashing into eachother though¹.

Safety cut-out

This particular breakdown stops the lift hill from working. If a dinghy is right at the top of the lift hill when the breakdown happens, it can stop right at the cusp of the hill. When the ride starts up again, this dinghy will start with a speed of 0 km/h, instead of starting with the speed of the lift hill (usually 8 km/h or 5 mph).

This may cause this one dinghy to not be able to complete the circuit, reversing, and flying off the track in reverse.

You can prevent this type of crash by making sure that the dinghies dont go very slowly at any point except for the lift hill and the station.

Long downward slope bug

Any drop with more than one steep downward slope may sometimes cause crashes, according to the RCT wiki.

That same wiki notes that this particular dinghy ride does have a tendency to crash.

¹: What usually happens here is that dinghy #2, filled with heavier guests thus going faster, catches up to #1 and bumps into #1 on the top of a hill in the track, transferring its momentum to it. This usually happens near the top of a hill as the dinghies are moving slowest, so the distance between them is smallest there. Dinghy #2 will then slide back down into the preceding valley. Dinghy #3 can then hit #2 at high speed as it's travelling fast in the valley, at which point both will explode.