Why do large blocks increase the probability of chain reorgs?
This shows the inherent flaw with the idea of increasing block sizes if the network cannot handle them propagating in a timely manner.

If the block sizes are huge and the mempools of the connected nodes are way out of sync, the full node will basically have to download almost entire block before adding to its chain and then transmitting this block to the nodes that are connected to it. This can consume a lot of time. If a miner mines a block h, they will generally try to build block h+1 on top of the block that they found. It is only after they receive block h+1 (before they mined it) will it know that they lost the 'race' for h+1. The miners will then reorganize its chain to the best height h+1 that they just received and mine on top of that. 

When block sizes are huge, the miner in above example, might receive block h+1 only after he has mined h+1. So the miner will start building on top of the block he has mined, being deceived that he was the first to find the solution to the block h+1. If block sizes are huge, this can extend for multiple heights, until the miner receives say a block at height h+6 before he mined the block at that height. So the miner will now have his chain reorganized based on the blocks he has received in the past. The full nodes that are closely connected to the miners, will also have to reorganize their chain for that to happen.