When food or other necessities are scarce, deliberately withholding supplies to create or worsen scarcity can harm the public. The concern is not every ordinary fluctuation in market prices but the attempt to profit from others' hardship.
Example: During a shortage, a trader hides a large stock of essential food in order to make supplies appear scarcer and sell later at inflated prices. Low-income families may suffer most.
الْمُحْتَكِرُ is an active participle connected to اِحْتِكَار (hoarding). The criticism concerns the harmful practice and the attitude behind it.