Comcast stock tumbled Wednesday after the cable and entertainment company said it expects to lose broadband subscribers in the fiscal third quarter as competition from fiber internet providers intensifies.

Comcast has faced growing pressure from rivals in both the cable and wireless phone markets. The price strategies adopted by some of these competitors, however, are "not rational," Chief Financial Officer Jason Armstrong said at the Goldman Sachs Communacopia & Technology Conference on Wednesday.

Due to aggressive fiber pricing, Armstrong cautioned that broadband subscriber losses for the fiscal third quarter will worsen year over year. However, fiscal-year subscriber losses should still improve from the previous period, he added.

"I think quarters are going to now look different within that," Armstrong said. "This particular quarter, I don't think it will improve year over year."

Shares pulled back after his comments to fall 6.7% on Wednesday. Charter Communications also dropped 7.7%. Both companies joined the ranks of the worst-performing stocks in the benchmark S&P 500 index.

Armstrong said fiber competitors are offering introductory rates of $30 to $40 a month for 1-Gigabit speeds-pricing he described as "not rational." With fiber buildout costs running from $1,500 to $2,000 per home, he said it is mathematically difficult for competitors to recover their investment at such low rates.

Management previously cited concern over broadband competition in an earnings call for their fiscal second-quarter earnings, which Comcast released on July 23.

For new customers, Comcast currently charges $50 a month for 1-Gigabit internet plan under a five-year price guarantee, according to its website.

But the fiber pricing point isn't the only dark cloud looming over Comcast. Armstrong said that while satellite internet hasn't hurt numbers significantly yet, the company expects it to become a real threat soon, particularly in rural and suburban areas.