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Hello everyone, Mark from Osoyoos, B.C. Canada here. I was wondering if anyone can help me identify the problem with my tree cactus falling over this year. It seems to be healthy but I am not sure what to do as it is crowding the plant below it, see pictures.

My questions are:

Should I cut some of the branches?
Is it getting too much water?
Should I feed it?
Up until this year the cactus stood 5.5 feet high and supported itself.

I'm not sure what the issue is. The plant looks healthy to me from what I can see in the photo. How much light does the plant get there? If the plant changes position throughout the day/often, I would think that it is 'looking' for sunlight.

I see two possible problems with your cholla cactus.
The first one is caused by not having enough light, which causes an elongation (etiolation) of the internodes. This will lead to weak and thin stem segments, which are more likely to topple, break off, or just lean toward where light is available. Then, once the stem does receive an adequate amount of sun light, it does thicken, which makes it even more top-heavy.
The second thing may have to do with the lawn. Usually lawns are watered regularly and fertilized. Once your cactus has produced a sufficiently large root system, it will tap into these additional nutrients, which results in rapid growth on the side of the plant adjacent to the lawn. This too will unbalance the plant and make it lean.
You can go ahead and prune off the branches that are not where you want them to be, or cut off whole stems or just top them. These plants are rather sturdy and nature designed them, so the stem segments break off easily, if an animal brushes against them and the spines dislodge in its fur. After some heavy rains, I have had whole plants of my large-padded prickly pear cacti fall over, almost uprooting them, because the soil was softened up by the rain and the roots could no longer hold the weight of the top. These plants also did receive irrigation water, which made them grow too vigorously. In their native habitat, they will have to make do with natural rainfall, and the soil contains a lot of rocks, which do hold the cactus in place.
I hope, that my long post will be of value and help you with the decision-making process. Your plant does look healthy by the way. If you do have a dry spot that does have ample sun light, you may want to plant a section or two there and you will most likely not experience the same problem.

Harald