Around here impatiens are perennial, but nevertheless:

They are a very heavy plant and tend to spread over time.
Depending on your weather and your ground you may be overwatering
and/or overfertilizing. If they grow too fast they don't have time to grow
strong enough to support their own weight.

Try cutting down on water and fertilizer. And water less often, but deep.

Aloha,

Maren in Hilo, HI