That's normal for heavy riders, although properly built wheels don't break spokes for the first few hundred thousand miles regardless of rider weight.

Bike companies make more money dealing with occasional warranty returns than they do investing in expensive Holland Mechanics stress relieving machines or paying for expensive hand labor to prevent your problem.

Spokes fail due to fatigue, with the number of cycles survived (about 750 per mile as a spoke passes the bottom of the wheel) dependent on magnitude of the variation and average stress. Magnitude of the cycle comes from the rider+bike+load weight with about 60% of the total on the rear wheel; obviously this is more for heavy riders. Average stress is high because parts of the elbows were never taken past their elastic limit during the forming operation.

Since they all see the same conditions they all fail at about the same time like popcorn kernels popping - first a few some time apart, then most of them in rapid succession, then the remainder.

You need to replace your spokes with quality butted spokes and properly stress relieve by taking the elbows past their elastic limit. You can squeeze near parallel pairs of spokes hard (gloves make that more comfortable), or wind them about eachother at the outer crossing using something softer like a plastic screw driver handle, old left crank, or brass drift.

Realistically, that means doing the job yourself or finding a reputable one-person shop to do the job. With the low end of the market buying $25 QBP wheels and high end mostly boutique the average shop mechanic doesn't get enough experience building wheels to do a good job in a profitably quick time so they compromise by doing a bad job fast. As some one not trying to make a profit building wheels you can take hours to get it right provided you have a little mechanical aptitude (about what it takes to adjust a front derailleur).