Breaking at the J bend is typically too low tension.
As the wheel is rotating, the stress is constantly moving from spoke to spoke. If the tension is low, the weight transfer has kind of a "running start" to flex the J.
If tension is higher, it doesn't take as much of a jolt.
Thing of getting hit by a punch or getting a shove.

Rears also take the pedaling stresses. You are basically trying to "unwind" 1/2 the spokes every stroke. (combine that with too low of tension and you may actually start getting a slack spoke with nipples starting to loosen.)
If you mash low gears going uphill, think of the stress.

You may also simply have a wheel without enough spokes for you.
How much do you weigh?
How many spokes?
How many "speeds".