Given that the OP is likely only to earn whatever miles may be on offer in Eurobonus or Miles & More for the sole TP-operated flight LIS-OPO which are only 172 miles apart - which may well be zero, particularly if, as I suspect, this is a bargain-basement fare - this does not look like a mileage-inspired routing.

Instead, my guess is that some online TA was able to offer a reduced fare on this "crazy" routing - but I am hoping that they may now re-issue the ticket on a more mileage-friendly routing (if it was issued on TP stock there is at least a chance that the LIS-LAD on TP may come into play).