When asking for the meaning of what might seem a phrase to you, it is important that you give the full sentence, if not paragraph, where you found this written.
" set in tumble" is NOT a phrase in English.
the full sentence would be, 'a cutlery set in tumble polish finish"
"tumble polishing" is a method of smoothing and polishing rocks, metal and glass, done with some abrasive material acting on the material, and lubricated with soap or some such.
I think the expression in question definitely is an English phrase which is wide-spread in Business English.
Stocks Set to Tumble amid Bear, Fed News
GM Stocs Set to Tumble
Bandwidth prices set to tumble as BT exploits Asian fiber frenzy - Company Business and Marketing
Proceeding from the following below excerpt from an article I am able to give an acceptable explanation of the expression “set to tumble”.
European Real Estate: Set to Tumble?
A moderate slowdown is more likely, says S&P Ratings, though Spain's sizzling growth raises concerns
Demographic, financial, and structural factors are powering the current boom in the majority of Europe's real estate markets. Nevertheless, double-digit house-price inflation cannot be sustained indefinitely, and legitimate reasons exist to fear that the boom may turn into a crash. We at Standard & Poor's Ratings Services consider a moderate slowdown is more likely for these markets, however.
My free interpretation of the expression in question is somehow or other given below.
set to tumble = turn upside the things – make a mess of things, confuse things, get into a tangle