Hey there Mike.
This is a simple situation of supply and demand. Back in February of 2022, HSAV suspended subscriptions for units. This means that now when someone wants to buy a unit, they're doing so from the secondary market. And as such, it created a premium to the funds net asset value. If subscriptions weren't suspended, if someone wanted to buy it and there was no secondary market action at the net asset value, the Authorized Participant (a bank typically) could issue a new unit at NAV and sell it to you.
Buying these funds at a premium to NAV always poses risk, because it relies on secondary activity and high demand to maintain that premium. The banking crisis caused a lot of fear in the markets, and retail investors likely thought there was a chance Canadian banks could go insolvent. As such, they sold off their HSAV, demand dropped, and it started to trade closer to its NAV.
Here is a video I made about a month ago discussing this. Head to the "understanding how ETFs work" chapter and it's going to explain the situation.
https://www.youtube.com/watch?v=p9cmsGXLFlY