"There were a few fundamental batting mistakes made by our left-handers," said Siddons. "I've spoken to two of them. They were trying to hit the ball to the leg side instead of clearing the front pad and hitting the ball back where it came from."
Although Mulder and opening bowler Duanne Olivier took the wickets, Siddons felt the biggest threat came from South Africa's spinners, Keshav Maharaj and Simon Harmer, who were South Africa's match-winners in the first Test at Kingsmead in Durban.
"Maharaj hasn't got a wicket yet but he's bowling really good quality spin so it's going to be a tough match from here on in. The ball is spinning quite a lot and at good pace so it's going to be a difficult day of batting."
Tamim and Najmul put on 79 for Bangladesh's second wicket and Siddons said Tamim "played really well" before playing around his pad to be Mulder's first victim.
"Tamim was maybe looking for a four to bring up his fifty and had maybe forgotten how he had been playing the whole innings. He had been playing beautifully straight and not putting his pad across. His innings was outstanding, taking the fight to the bowlers.
"Quite a few of the batsmen looked comfortable and played some good shots."
Maharaj, man of the match for his bowling in South Africa's 220-run win in the first Test, top-scored for South Africa with a career-best 84.
He hit his runs off 95 balls with nine fours and three sixes as South Africa added 175 runs to their overnight 278 for five.
Bangladesh left-arm spinner Taijul Islam took six for 135 - the tenth time he has taken five wickets or more in a Test innings.
Maharaj raced to his fourth Test half-century off 50 balls with four fours and three sixes.
"I've been working hard in the nets after hours," said Maharaj of his batting.
Maharaj and Harmer both proved fairly expensive, with Maharaj conceding 42 runs in 11 overs and Harmer 31 in seven.
"There were a lot more shots," said Maharaj. "I don't know if it's a case of not trusting your defence or trying not to allow us to settle but there were a lot of chances.
"In terms of the pitch compared to Kingsmead, the ball is turning quite viciously and hopefully we can utilise that tomorrow."
Maharaj said Mulder deserved some success after being criticised for playing a minimal role in the first Test. He scored 33 in a seventh wicket partnership of 81 with Maharaj before striking with the ball.
"The partnerships that he has been involved in have been very good for us and he showed that again today. He showed his value with the ball and he adds balance and variety to the attack."