In fact, Old English had multiple conjugation patterns even for weak verbs (verbs ending in a dental suffix), and it seems that the differences were relevant to the development of irregularities in the past-tense forms of some verbs. Send and end did not originally form their past tenses in the same way.
Send, bend, rend had contracted past-tense forms already in Old English: the OED shows forms like sende, bende, rende as being used in Old English, and marks sended, bended as Middle English, not Old English, past-tense forms. (The past participle shows different behavior.)
The important thing is the division between Class I weak verbs and Class II weak verbs. Class I weak verbs contained a derivational suffix [(i)j~i] in Proto-Germanic. In the past tense, this was subject to deletion after a heavy verb root by the time of Old English:
In the past tense forms, a mid vowel (phonetically schwa) appears before the preterite /d/ in verbs with a light root but not in verbs with a heavy root
(p. 510, "The Dental Preterites in the History of English", by Aditi Lahiri)
The verb bend is from a Class I weak verb, bendan
The Oxford English Dictionary gives the following summary of the historical forms of the verb bend:
Forms: past tenseOE–ME bende, ME–15 bend, ME bente, ME– bent, 15– bended. past participleOE bended, ME y-, i-, ye-bent, ME–15 bente, 15 y-, i-bente, bende, ME– bended, bent
The OED's Proto-Germanic etymology shows -ja- in the infinitive, a characteristic of this class of verbs:
Germanic *bandjan, < bandjâ- ‘string, band’
The Old English past-tense form bende shows vowel deletion/contraction and coalescence of the dental suffix with the final consonant of the root. But in the Old English past participle (ge)bended, deletion did not originally occur because the /d/ was word-final rather than being followed by a vowel as in the past-tense form. In Middle English, there seems to have been influence between the past participles and past tense forms that resulted in all verbs of this type eventually having a leveled paradigm. (Aside from a general tendency towards leveling these two forms, the loss of schwas in ME would have eliminated the original phonological motivation for having contraction in the past tense but not in the past participle.)
The process leading to the devoicing that we see in the modern past-tense and past-participle form bent (and in a number of other verbs) is more obscure. The linked Lahiri paper attempts to explain it, but I don't fully understand the explanation given.
The verb end is from a Class II weak verb, endian
Verbs of this class originally had a derivational suffix containing a vowel that was *ō in Proto-Germanic. The OED etymology of end shows this characteristic o in the given Proto-Germanic etymon:
< Old Germanic *andjôjan , < *andjo- end n.
For some reason, verbs of this class have an -i- in the Old English infinitive form, but that's not relevant to the development of the past tense.
The past tense of Class II weak verbs was originally formed with this vowel before the d. The vowel was regularly subject to shortening because of the lack of stress on it (resulting in Old English past tense forms ending in -ode, with short o) and it was frequently subject to weakening/fronting/raising (resulting in past tense forms with ed(e) and id(e)). But this ō-derived vowel was not subject to deletion as early as Class I verbs were subject to deletion of their [j~i]-derived vowel.
Of course, verbs with a regular weak conjugation in Modern English don't always come from Class II weak verbs (many regular weak verbs ending in d or t come from either borrowings or part-of-speech conversions after the Middle English period).
And weak verbs with irregular past tense/participle forms don't always come from Class I weak verbs. However, this division is important for understanding how verbs ending in d and t with irregular weak past tenses developed, and also for understanding how verbs ending in other consonants that show irregular vowel shortening in the past tense (e.g. keep/kept).