Could be the hops. I'm not familiar with Ivanhoe, but I've had beers that are heavily C-hopped that have a huge grapefruit presence. Don't know if Ivanhoe has any pineapple tropical fruit character. Did you pick up any of that at all when smelling the dried hops?
Another possibility might be oxidation. I've occasionally picked up HINTS of pineapple along with sherry or cardboard notes in aged barleywines or other big beers. But those were just hints, not tasting like pineapple juice. Of course, sensitivity & perception levels of various flavor components vary with the taster. Wouldn't expect this though in a newly fermenting beer.
Another possibility is esters from the yeast. It'll be interesting to see if this flavor stays, develops further, or goes away as the fermentation & conditioning continue. What is the alcohol character like at this point?