Basically a regime of very low interest rates alongside tighter regulatory requirements creates things that haven't been seen before. Negative swap spreads for example mean that investors consider credit risk of swaps lower than US treasuries, which are supposed to be the golden unattainable standard of safety. The reason for that is that swaps are collateralized, that is counterparties in a swap are obliged to give each other money they are owing pretty much each day so you are on the hook just for 1 day market movement. Not so with treasuries and faith in US treasury seem to down.

Negative corporate bond inventories happened because new rules make it very burdensome for banks to hold corporate bonds because of reserve capital requirements. So bank sell their inventories and because there are many non-banking financial institutions that seem to have an appetite for corporate bonds, banks sell even more bonds than they have (being "short").

For the same reason synthetic credit is popular. Instead if holding an asset (corporate bond in this case) you recreate synthetic position with derivatives. For example, if you buy a right to buy a bond at a certain price and at the same time sell a right to sell a bond at the same price, it is essentially the same as if you bought a bond (bond forward strictly speaking, but we will omit the difference here).