Personally I always keep my single car rollercoasters on "Wait for Full Load", as I suspect that these crashes are indeed caused by fuller, heavier trains catching up with the slower, empty ones. While I'd suspect these are more likely after safety cut offs (especially with multiple lift hills), don't forget that on long rides with slower sections, that one full train can easily end up going 50-100% faster over parts, more than enough to eventually catch up given enough time/short releases between trains.
For coasters like the reverser though, there's always the possibility that you just got unlucky with 6 very heavy guests putting the train juuuust over the speed limit to crash, combined with some possible random variation in coaster speed (which I swear does happen occasionally, such as with brakes). You'd need to be pretty unlucky to get it, thus the delay, but if it's possible, it'll happen eventually.
As a final aside, another personal way of avoiding such crashes (in addition to waiting for full load) is to make sure the trains travel straight from the station to the lift hill, with no space for another car to dispatch when there's no block sections (and no second lift hills). That way a train can't leave the station, get stuck on the lift hill, and have another train come up right behind it on the lift hill. Even if they don't directly collide and crash (like on a mid course lift hill), the trains collision often slows them right down after the lift hill (from them still "colliding"), or requires a load of bumping to get a stalled train onto the lift hill (with similar problems).
That, and sometimes the game just calculates something funny that causes a crash. That's how I've had rides crashing when one's going through a loop and the other is on track going through that loop, a ride with no speed issue just... Slowing down, and the best one, a stand up coaster flying off the track. On a complete circuit. In the original RCT.