The easiest explanation for the whole chart is "Software dev is more reactive to the monetary policy environment than most other industries, because more of it is funded by new capital investment -- whether VC money or otherwise -- instead of ongoing operations of stable firms compared to most other industries."
Trying to add other explanations is fun, but there's not a lot of evidence for any of them, or even that more explanations are needed.
Sec 175 especially hurt small to mid-size tech companies (especially fast growing ones...) which I imagine amplified the problem across the board.
You mean the change from deductible as a current expense to five-year amortization under changes to Sec 174? In an easy money situation, the way it increases tax liability in year one while decreasing in year 2-5 is not a big deal, it becomes more of an issue as the cost of financing goes up, so the main effect is to increase the already-high sensitivity of software development to tightening credit.