If you have spasticity combined with LMN weakness, it can complicate things as well. In a healthy person, the brain sends many many signals to your muscles to maintain balance, and also receives feedback from those muscles in order to help maintain balance. By making constant adjusments to a whole network of muscles, a person can remain upright without even thinking about things.
Unfortunately, it's different for folks who have ALS/MND. While your brain is working as hard as it possibly can to keep you in a chosen position, it's likely only getting a few signals through to your muscles and only getting erratic signals back. As people said above, it's going to take more awareness and care on your part to be able to maintain balance or make sure you are in a safe position if tipping over is happening more frequently.