That's fine if someone does not feel safe. They should not tip over if they are properly secured. Maybe they need a different/better method of securing the
ECV or chair if this happening. After all anyone on the bus can get hurt if the ECV is becoming loose during the ride.
When the ECV user is unable to transfer off of the ECV on the bus, are s/he and the ECV belted in securely enough that, even though the ECV might tip a little, neither the user nor the seat would not move very far or fall over?
The technical term for this is 'securement'. ECV/wheel-chairs/etc. will have recommended securement points that are engaged by the ratcheting tie-downs connected to similar points on the bus. Generally speaking, once secured, an ECV is not going to move much relative to the bus, and the next concern is securing the ECV passenger to the ECV and/or bus.
This leaves two possible problems, such as...
Sure, they should not too over. But gravity is a funny thing. It has surprising laws and properties.
Gravity remains more or less a constant, but I know what you are going for. Force, in physics is how mass in motion affects the world around it. A seatbelt for instance must be able to secure it's payload against over 5000lbs of force. That's how 'heavy' a 180lb person becomes against the seatbelt when a car hits a stationary object at 30mph. The ratcheting tie-downs are made of similar material.
So... if the ECV passenger wil be secured to the ECV and the ECV secured to the bus, a bus driver has to determine whether the securement system will handle the total burden. Adding a 120lb passenger to a secured ECV is a very different matter than if the rider is 200 or 300 lbs, from a purely Newtonian standpoint.
The second issue is that not all passengers are the same, in seats or sitting on an ECV. The rest of us don't get seatbelts on most buses, we're trusted to secure ourselves with handholds. Often though someone who needs help walking might need more help in general and the handholds available to someone sitting in a secured ECV may not be as good as those available to a person seated in a proper chair.
All of this is the sort of thing the driver must consider when setting his policy for who gets to sit in their ecv vs. who moves to a seat. Often, drivers will just decide that a blanket policy of everyone transfers to a seat is just easier to manage, and the safer, if less convenient, option.
Very often I find that people tend to crowd into every space in those lines and have no problem jumping in front of a wheelchair. I have been hit and my chair has been damaged by people using it to push themselves in front.
I still find people that doubt this actually happens to any problematic degree. I'll hear, "I'm sure it happens occasionally..." sort of things.
I installed cameras on my friends powerchair as a part of a collision avoidance system I'm developing and I have saved over 1000 hours of video as test data so far. I can pull probably 100 examples of this behavior, sometimes 10 or more in a single day.