This is a very good question. I’ve written a detailed answer there: Network partitioning - #2 by fabrice Let me know if something is unclear.
The short answer is: there is no risk of forking on Algorand even in case of network partitioning as long as something like 80% of the (online) stake is honest.
The very high-level reason is that all nodes know how much stake is supposed to be “online”, aka participating, as this information is written in the blockchain.