Use 'mount up' to show something is slowly rising, like stress or work, not for sudden changes. Pressure at work started to mount up during the holiday season.
Pro Tip 2/3
Negative Situations
'Mount up' often suggests a negative buildup, like problems or debts. Worries about the test began to mount up.
Pro Tip 3/3
Active Resolution
When things mount up, it implies a need for action to address the increase. I need to sort out my tasks; they're mounting up.