It was very much an edge case, with one of Whitlam's senators on leave and recent changes to territory rules giving additional senators to the opposition party (as I recall ...) the ability to block supply appeared suddenly out of the blue.

Whitlam did move to call an election (rather than be sacked) which likely would have removed the blocked supply threat as he was at the time an extremely popular PM in Australia (loved by the common masses, despised by many elites) .. and when attending the Queens Repreresentative (the Governor General) to advise about calling an election .. he was removed by the G-G.

Strictly speaking the "as intended" outcome should have been to resolve a looming (not yet happened) supply crisis by allowing the people of Australia to vote, instead the government of the day (Whitlam's) was removed on a technical reading against the spirit of intended resolution.

There's a peer comment here that linked to a 2020 article on the finally released royal correspondance that's worth a read. The US influence angle has merit also, they had weight in the game for sure, how much and whether it tipped the balance is debatable.

Literally reams of contraversay here, the G-G acted autonomously and likely to save his own neck as Whitlam intended to replace the G-G, additionally many outside powers (the UK and the US) were whispering in the ears of those with levers to pull seeking to dump Whitlam; he was returning real power to the people, providing socialised health and education to the masses, asking questions about the role of secret American bases on AU soil, etc.

This was, indeed, extremely serious stuff: https://www.youtube.com/watch?v=A4jfR2u_9Kk