Unable to switch DAG:
If your miner does not support a switch of DAG, it will result in 100% of hashrate rejection without earnings. You need to restart it to be normal.

Example:
 AntMiner E3 can switch from ETH to ETC via onebutton switch without reboot.
 AntMiner E3 can switch from ETC to ETH via onebutton switch with reboot.

What is DAG:
In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG or dag /ˈdæɡ/ (listen)) is a finite directed graph with no directed cycles. That is, it consists of finitely many vertices and edges (also called arcs), with each edge directed from one vertex to another, such that there is no way to start at any vertex v and follow a consistentlydirected sequence of edges that eventually loops back to v again. Equivalently, a DAG is a directed graph that has a topological ordering, a sequence of the vertices such that every edge is directed from earlier to later in the sequence. In Ethereum, each epoch create a DAG using the DaggerHashimoto algorithm and combining Vitalik Buterin's Dagger algorithm with Thaddeus Dryja's Hashimoto algorithm.