The briefness of the entering tones is caused by the stop codas -p, -t and -k.
Examples (Wiktionary)
Mandarin
第三组读塞音的方言极少，例字亦寥寥无几；南海九江和斗门镇各有八个例字，而开平1、台城2及鹤山只各一孤例。
第三組讀塞音的方言極少，例字亦寥寥無幾；南海九江和斗門鎮各有八個例字，而開平1、台城2及鶴山只各一孤例。
dì － sān zǔ dú sè yīn de fāng yán jí shǎo ， lì zì yì liáo liáo wú jǐ ； nán hǎi jiǔ jiāng hé dǒu mén zhèn gè yǒu bā ge lì zì ， ér kāi píng １ ， tái chéng ２ jí hè shān zhǐ gè yī gū lì ．
There are extremely few dialects that pronounce the third group as stops, and example characters are very few in number; Jiujiang, Nanhai and Doumen Town each have eight examples, and Kaiping 1, Taicheng 2, and Heshan each only have one example.