By default Chrom becomes wed to Sumia relatively early in the game. Marriage has the support rank of "S" on the Support menu. When inspecting Chrom's available support options I saw that there are 3 other possible "S" characters. Can Chrom marry someone other than Sumia?
Yes. The tricky part is is that Chrom needs to have formed a relationship after chapter 11. At that point Chrom will marry one of; your Avatar (if female), Sully, Maribelle, Sumia, or Olivia. Whichever of those 5 he has the highest relationship with, and is not already married off. If he married Sumia in your game(s), it is simply because he has spent more time gaining relationship ranks with her in battles than his other suitors.
Olivia is the trickiest to get him to marry, as you have but one mission to recruit and have Chrom woo her (just have them constantly support each other in battle, and have her dance for Chrom every turn).
I once had to re-start, because while I was trying to get my avatar to marry Donnel, Chapter 11 would have pushed them to S-rank, but because my Avatar was A-rank with Chrom, it forced the marriage between Chrom and my avatar after Chapter 11.
Interestingly, if all of Chrom's suitors are married off and Olivia has no contact with him in chapter 11 (or she dies), Chrom will marry some random village girl.
(Chrom is the only character who's marriage is determined at a fixed point. All other characters can be married off by having them support each other in battle at your leisure)