to be about to do something - לעמוד לעשות משהו

But as Max was leaving ("stood to leave") אבל כשמקס עמד לצאת

The expression לעמוד לעשות ("to stand to do [something]") means "to be about to do [something]".

I was about to call you when you showed up. עמדתי להתקשר אליך כשהופעת.
I was about to finish painting (the painting work) when all the paint spilled on the floor. עמדתי לסיים את הצביעה כשכל הצבע נשפך על הרצפה.