Beware a cascading 'flash crash' in stocks as everyone packs into the market's most popular names, JPMorgan chief stock strategist says

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• A stock rally unwinding could come with no warning, JPMorgan's Dubravko Lakos-Bujas said in a webinar.
• Concentration is so high that if one large fund begins pulling out, it could trigger a broad market fallout.
• Cracks are already showing as Apple and Tesla stocks slide.

The equity rally that's taken stocks on a five-month tear could rupture with no warning, Dubravko Lakos-Bujas said in a JPMorgan webinar on Wednesday. 

Though it's unclear when this could happen...