After the v7 fork, the Monero hash rate went down, but some opportunistic Monero forks such as Monero Classic had hash rate shoot up. Why is that so ?
When Monero forked to v7, it changed its proof of work algorithm. While it previously used Cryptonight, it now uses a slight variant (technically, variant 1) of Cryptonight, which should be typically slightly slower to compute.
After the fork, miners would were not able to update their hash algorithm were left unable to mine Monero. While patches were made to most known mining software, one major type of miner could not update: ASICs.
ASICs are hardware made to run one particular piece of code fast, and nothing else. ASICs were made to mine Cryptonight, and were able to do it much faster than CPUs and GPUs, leaving normal users out in the cold. When Monero forked, these ASICs were left unable to mine Monero, causing the overall network hash rate to plummet, restoring viability to everyone else.
However, these ASIC miners could still mine other forks such as Monero Classic which are still using the legacy proof of work, leading to high hash rates. However, since they are mining incompatible algorithms, their hash rate can no longer be compared. Indeed, the average time to compute a "variant 1" hash is larger than the average time to compute a legacy hash, since ASICs exist to compute legacy ones very fast.