There are many causes of deforestation, some are immediate, like a massive construction, some are slow, as the forest degrades, due to temperature rise and an increase in fires.
Forest fires: Each year, fires burn millions of hectares of forest worldwide. Fires are a part of nature but degraded forests are particularly vulnerable. These include heavily logged rainforests, forests on peat soils, or where forest fires have been suppressed for years allowing unnatural accumulation of vegetation that makes the fire burn more intensely. The resulting loss has wide-reaching consequences on biodiversity, climate, and the economy. Fires may start accidentally or be started in order to clear the land for farming.
Illegal and unsustainable logging: Illegal logging occurs in all types of forests across all continents – from Brazil to Indonesia – destroying nature and wildlife, taking away community livelihoods and distorting trade. Illegally harvested wood finds its way into major consumption markets, such as the U.S., and European Union, which further fuels the cycle.
Fuelwood harvesting: Over-harvesting for domestic use or for commercial trade in charcoal significantly damages forests. Forests are now being cut down to make wood pellets.
Mining: The impact of mining on tropical forests is growing due to rising demand and high mineral prices. Mining projects are often accompanied by major infrastructure construction, such as roads, railway lines and power stations, putting further pressure on forests and freshwater ecosystems.
Climate change: Forest loss is both a cause and an effect of our changing climate. Climate change can damage forests, for instance by drying out tropical rainforests and increasing fire damage in boreal forests. Inside forests, climate change is already harming biodiversity, a threat that is likely to increase. Increasing temperatures also bring new pests such as invasive borer beetles
Reference: https://wwf.panda.org/discover/our_focus/forests_practice/deforestation_causes2/