Here's my best explanation (which might not be accurate so someone please let me know).
There are three main differentiators between Bicoin's difficulty algorithm and Sia's that are at play here (there are more differences, but these seem to be the most important):
1. Difficulty adjusts every block, rather than every 2016 blocks (~2 weeks).
2. Difficulty adjustments are limited to 0.4% per block, while Bitcoin's adjustment is unbounded.
3. Rather than adjusting difficulty to meet the targeted blocktime on a go-forward basis, Sia looks at time since genesis and adjusts blocktimes to get back on track.
https://www.reddit.com/r/siacoin/comments/7rz606/update_on_the_block_139000_hardfork/The above post explains it much better than I could, but in the context of the A3's hitting the network:
1. A3's start going online and the hashrate increases much faster than the difficulty can adjust for.
2. Difficulty increases, and continues to increase, even after hashrate flattens out to slow down block generation until we are in line with where we should be relative to genesis.
3. Once we are closer to aligning actual block solves to when they should be happening, difficulty starts to adjust back down so we don't overshoot and fall behind schedule.