“Correlation” is about the directions of changes, not the absolute values 
They can, and one should plan for a worst case scenario.
I personally calculate with about a 50% margin requirement. Which is the worst case expectation essentially. Not happened yet for broad index funds at ibkr as far as I know.
At 150% in an all stock portfolio and different maintenance margins, margin call would be triggered like this:
| Maintenance margin | Market decline before margin call |
|---|---|
| 50% | -33.3% |
| 40% | -44.4% |
| 30% | -52.4% |
| 25% | -55.6% |
| 20% | -58.3% |
Looking at a GFC scenario, and assuming you can pay back some margin as it drops, you risk liquidation at anything more than ~30% maintenance margin.
And actually IBKR did put maintenance margin at 30% for broad index funds in 2008. So you were really just about safe at 150% stocks back then. But barely.
So one is being risky, but not completely out of line. Personally too risky for my taste.
If you of course use more uncorrelated asset, the game changes a lot and you have a lot more leeway. Hence why I am personally very comfortable wih my own ~150% being comprised of 50/50 stocks/non-stocks (although I only even borrow half of it and the rest is in stacked funds that sel rebalance on the way down, preventing ballooning leverage).
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