Finlay started the flurry in the 78th minute with a one-timer off Hector Jimenez's cross. A minute later, Meram weaved through the defense to even the score, and Arrieta flicked the winner past the goalkeeper in the 82nd.
The Crew (12-10-10) solidified their hold on the fifth-and-final MLS playoff position, extending their lead to six points over Toronto FC, the nearest challenger.
The late collapse was a crushing blow to Union's playoff hopes when a win would have moved them within a point of Columbus.
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