"sinking" concrete work?

I rebuilt a trough drain (with insight from this group!) early last summer. Aesthetically it turned out VERY nicely, and so far through our brutal winter in Northern Ontario, has not cracked or split anywhere.
However - I've noticed that in relation to the garage pad (it remains level all the time), the rails of the trough have "sunk" about 3/4" on one side. The rest of the trough remains level. Of course, this is the /far/ side of the trough away from the drain :(
Any thoughts on why this occurred? Where the drain met the pad, I ran 1/8" buffalo board between. The rails are welded together in a square fashion via 1/4" rebar running crosswise, so one rail could not move out of context of the other. Are there odds it will "come back" some in the spring when the ground thaws? The driveway (ashphalt) heaves notably in the winter and goes back down in the summer....
I guess I'll have to wait for spring after things thaw out to see where it lands - my only /real/ concern is with the propogation of water. It not being lower on the far side means the water will have to build a lot higher before it runs towards the drain, and likely some will pool on that side when the water stops...
grrrrrrrrrr :) any ideas?
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