Ten minutes prior to the close of any trading day market makers aggressively take steps to reconcile the books. They'll buy shares prior to the market session and then unload those unsold in the closing moments of the session. What you're seeing is the trail of what I'm saying just above, I'm betting.
There's nothing odd about it but I've seen this for many years and have known enough brokerage staffers (market makers) to recognize what's going on.
More and more I'm liking UNIT. I've got a major $$ windfall coming in from NGL in several weeks. UNIT is looking like my #1 pick for accumulation.
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