Deprioritization can happen on basic unlimited plans with major providers but is a more common issue with prepaid plans or mobile virtual network operator (MVNO) wireless plans. "What are MVNOs?" you're probably wondering. MVNOs are like any other carrier, except they don't have their own network—instead, they just use other networks. For example, Visible Wireless uses Verizon's network, and Metro by T-Mobile uses (you guessed it!) T-Mobile's network.
MVNOs are kind of like the renters of the wireless network space. The big four carriers own the networks and the MVNOs rent space on those networks. The plus side to the arrangement is that MVNOs typically offer cheaper cell phone plans, with the best prepaid mobile carriers like Mint and Visible offering high data caps for less than half the price of a phone plan from a major provider. On the negative side, MVNOs suffer from deprioritization.
While using an MVNO, your signal can suddenly disappear in congested areas. This happens because whatever network you're using gives priority to its own customers.
For example, let's say you, a Visible Wireless user, attend an MLB baseball game with your friend, who is a Verizon user. You struggle to get a single text out during the game, while your friend has no problem posting updates to Twitter. Since you're both on Verizon's network, shouldn't you have the same signal? Nope! Since Visible Wireless is an MVNO, its data speeds can be deprioritized.