Front overturns have occurred, but are rare. Approved ROPS devices are crush-tested and certified. These tests are specific and determine if the structure around the operator station remains intact in an overturn. Without performing these tests, you do not know if a homemade structure will protect you during a tractor overturn. Reference - Use Tractors With ROPs to Save Lives.
Rollovers can occur even on level ground if an operator approaches a turn too fast.
All of the above. Limiting speed while turning on level ground, on hills or on rough ground is an effective method of preventing side overturns. One should avoid steep hills whenever possible. Providing ballast for rear tires provides a more stable base for the tractor. When you turn around to look at the disc you are pulling you inadvertently steer the tractor to the right.