Warthogs are not diggers on the scale their shelter requires. They occupy holes excavated by aardvarks, enlarging and maintaining them, and a burrow may be reused by successive occupants for years. The entry technique is consistent: an adult approaches, turns, and reverses in, so that it ends up facing outward.
The reason is defensive geometry. A warthog reversed into a burrow presents its head and lower tusks to a predator at the entrance, in a passage too narrow for the attacker to get past. Emerging is also faster and more explosive from that position, and warthogs leave a burrow at speed rather than cautiously. Piglets typically enter head first and are let in ahead of the sow.
Two details are commonly muddled. The warts are not tusks but pads of thickened skin and gristle that cushion blows during male fights, larger in boars than sows. And the upward-curving upper tusks are blunter than they look; the shorter lower tusks, honed against the uppers each time the jaw moves, do the cutting.