Indexing
I have a 95 Dependable Care Maytag, super capacity washer, and mine too will index at times under a heavy load, even when ex large water setting is used.
If I was doing the same wash load, I would use the highest water level. Jeans are a heavier load, and require a higher water level for them to circulate for a proper rollover. You used a medium water level, and making the machine to work harder during agitation, causing the tub to index in the process.
Just my observation.
Rich